Skill Build:

Nethertoxin: 1/2, 3/4, 7, 8
Poison Attack: 1/2, 3/4, 5, 9
Corrosive Skin: 10, 12, 13, 14
Viper Strike: 6, 11, 16

for more information about Vipers skillset and stats please look at http://www.playdota.com/heroes/84

If you want to lasthit well from Lv1 choose Nethertoxin at Lv1,3 (in a solo lane / solo mid) or if you prioritize harrasment choose Poison Attack at Lv1,2 (in a dual lane / top or bot).

For dual lanes you should pick the long lanes because you need long paths to orb walk a enemy down.

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Strategy:

Frequently harras the lane enemy (single or dual lane - but concentrate on the weaker laner) until Lv5 - with Lv3 Poison Attack you can start to increase the pace and go for a kill with orb walking. never just autocast your skill unless you fail to stay on top of things.

After Lv6 you should try to roam around ganking - throw a Viper Strike and orb walk the enemy hoping for a allied stun or slow.

Remember: You are not a carry but a very good ganker - so don't stick to your lane and farm for luxury items, but try to gank the shit out of your enemy and the luxury items will come as a bonus if you succeed.

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Orb Walking:

Press the Hotkey of your Orb Skill (C for Poison Attack)
Click on the target
After the Projectile comes out immediately cancel the animation-backswing through Right Click (moving)

Choosing the direction of where to Right Click (move) is crucial for good orb walking, it's easy if the enemy is fleeing - so you only move after him. If he charges you, you should try to circle him, instead of backing off to reduce the time spent turning. You can use this technique even without a Orb Skill, it's called Animation Cancelling then.

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Starting Items: there's no single set of starting items - you have to pick the right ones for the situation!

Magic Stick, very handy item which builds into Magic Wand with 3 Ironwood Branches at a pathetic low price.
Ring of Basilus, if you don't like Magic Stick/Wand this is a viable alternative as you have really low starting int and armor. When you have >31 int (level 9~14) you can break it, sell the Ring of Protection and make a Oblivion Staff for Orchid Malevolence. Be sure to keep it on Hero Only armor unless you're pushing a Tower.
Ancient Tango of Essifiation, get more if you face damage over time spells, but buy at least one stack.
Healing Salve, if you expect to face hard comboers (Tiny f.e.) with high early burst damage (or fail to orb walk or judge the situation alot...).
Clarity Potion, get more if you face mana draining people, but save one up to have enough mana to Viper Strike at Lv6!
Ironwood Branch, to fill up empty slots and use up leftover money, at least get one to ensure a Good Game :)
Gauntlets of Ogre Strength, if you expect nukes and stronger comboes.

Core Items: these are the items you should at least have before thinking about anything else...

2x Bracer, you need a good health pool rather than anything else but they also give a little bit of mana pool increase which is nice to have
Magic Wand, gives some burst regen, stats and spell detection (Kunkka's Torrent, f.e.) for a very cheap price - simply a very handy item OR

No Empty Bottle?! Well, Viper really doesn't need that much mana: he has a orb skill, 2 passives and his ult. But as every solo mid hero he can use it to rune-whore, no problem with that but it's certainly no Core Item as Viper doesn't neccesarily need to go solo mid. He actually does way better in the long lane because there's no cliff to flee into the fog.

Phase Boots, movement speed is more important than attack speed for orb walkers and phase helps alot with chasing OR
Power Treads (Str), not the best alternative to Phase Boots but has it's viability with the bonus hp and if you want to buy Lothars Edge, since having two Zero-Collision abilities is a waste.

No Boots of Travel?! While they give the best movement speed bonus they're pretty expensive compared to Phase Boots and you won't be lane hopping like Tinker or pushing like Naga Siren anyways (Please consider; is 1 slot from the Teleportation ability really needed? Do you need that much movement speed that you pay 2200 for these two boni?!).

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Luxury Items: after a few successfull ganks you should think about what to get afterwards...

Empty Bottle, usefull item since you should roam alot after Lv5/6 and together with wards or allies like
Nerubian Weaver (Watchers), Treant Protector (Eyes in the Forest),
Rattletrap (Rocket Flare), Beastmaster (Call of the Wild) or Visage (Raise Revenants)
Lothars Edge, nice with backstab damage (good synergy with Nethertoxin) and a escape mechanism for a pretty low price, but not really nice when you already have Phase Boots
If you want to buy Lothars Edge, get Power Treads instead of Phase Boots. Good choice if you lack the micro for... OR
Necronomicon (Lv2/3), great synergy with Vipers overall skillset
While you slow the enemy your Demon Mages can easily hit him, the mana burn gives you some nice initial burst damage to let Nethertoxin set in; The movement aura helps you stay at your target and the true sight helps alot to kill those scaredy invis cats! OR
Orchid Malevolence, for Soul Burn (amplify Viper Strike) and mana pool/regen, the slight damage bonus isn't bad either.
Heart of Tarrasque, for more tank ability to last long enough for Nethertoxin to kick in...

NO Manta Style!! Images don't benefit from Nethertoxin thus doing rather pathetic damage - the only thing this item gives you that you can use is the stats and the ms bonus which other items provide more cost efficient.
(See http://www.playdota.com/forums/showthread.php?t=3910)

No Monkey King Bar?! While it gives nice damage, mini stuns and True Strike it's too situational - the usual argument that people just Town Portal away from you is also silly since those people just wasted 135 gold and alot of time unless they're Tinker and additionally you usually aren't alone - so how about asking a friendly ally for a stun? If you miss hits too often - go for it but don't just get it for the damage.

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Best Allies:

Huskar (Life Break + Nethertoxin) best combo ever to down enemies fast, also Inner Vitality is a strong heal.
Abbadon (Aphotic Shield/Death Coil + Poison Attack) makes it easy to orb walk someone down safely and play very aggressively.
Ogre Magi (Multicast + Fireblast>Ignite>Bloodlust + Poison Attack/Nethertoxin) best support for you with every single one of his skills! Multicasted Fireblasts give huge burst damage and a stun, Ignite gives a nice slow and dot damage and Bloodlust gives you a movement speed bonus too!
Bloodseeker (Stygwyr's Thirst/Rupture + Nethertoxin) slight desynergy because of slow of ruptured enemies but Stygwyr's Thirst makes finding low hp/lone heroes very easy. And Rupture's initial damage is good enough to start of Nethertoxin plus the enemy will usually stand still instead of running.
Naix (Feast/Open Wounds + Nethertoxin/Poison Sting) easy freekills in a lane as of lv5 Viper, lifeleech from Open Wounds? yes please! And Feast is really nice to get the hp of a enemy down to do Nethertoxin damage.
Zeus (Arc Lightning/Lightning Bolt/Thundergod's Wrath+Static Field + Nethertoxin) Static Field ensures high nuke damage throughout the game, thus giving good Nethertoxin bonus
etc. basically you need a ally with good burst damage, to get the enemy low enough for Nethertoxin to do good damage and/or a ally with good disables, for easier orb walking.

Worst Enemies:

Image Heroes - you lack the AoE damage to identify the real hero, if needed be get Shivas Guard.
Strong Disablers - heroes with several disables can keep you in check while the rest of your team dies, a Black King Bar can help in this situation - especially against a Doombringer that hates your guts!
Silencer (Curse of the Silent/Glaives of Wisdom/Global Silence), you have NO spell to break Curse of the Silent until lv6 - he will rape you even in a dual lane. Get a Preservance asap to counter regen his Curse of the Silent. Try to get a lane switch ASAP with a spell spammer, since he will have a very easy time against Silencer.
Tinker (Laser/Rearm), simply a stronger laner than you - his Laser is pure damage (not reduced by Corrosive Skin) and makes your orb walking fail alot. Orchid Malevolence and/or Monkey King Bar and allied ganks are a must against him. If there is a Nevermore in your team try to switch with him, he can at least nuke Tinker and denies way better than you. Otherwise pray for active and smart allies.