ONEATATIME's Guide to Faceless Void
1) I'm not quite sure of what you wrote ; the DPS varies quite extensively between level 1 and level 25, from a 603 gold inventory to a potentially 25+K worth inventory. If you were talking about the +20 stats from MY point 1), it's also something to take into account ; these +20 stats, when talking about EHP, adds +2.8 armor and more importantly, +380 HP. We all know how Void doesn't have that much HP.
2) Well, the EDPS concept like I use includes all forms of damage reductions, be it armor, damage block or evasion, or even the enemy's HP regen when it becomes significant, so you could get t = HP/EDPS.
3) I didn't say PMS or Vanguard were useless on melee heroes ; the damage block improves their EHP extensively, though it highly depends on the enemy's damage, your current level of armor and the possibility to use evasion.
What I said was that the +6 HP/s regen of Vanguard is negligible at improving your survivability when fighting heroes ; to give you a better idea, it's like getting lifesteal on Crystal Maiden or Undying. On these heroes, unless you've got a very special item build, you simply don't do enough DPS to get enough HP regen through lifesteal to make it worth on them.
Another thing showing that the regen is negligible is the "6-10 seconds rule" ; 95% or so of the teamfights are finished in a matter of 6-10 seconds after initiation. Assuming you're Void, you then have 1000 or a bit more HP in mid game, and that means if they needed 10 seconds of continuous fire on you to bring you down, then the EDPS is something of the order of 100+ HP/s, clearly dwarfing Vanguard's +6 HP/s.
Under focus fire, a mere 1-2 seconds of extra survivability is often enough to let your team do their job or break the chainstun, and that's the job of HP, armor, evasion, magic resistance/immunity, not Vanguard's HP regen. 1-2 seconds is also the time needed to frequently finish off an enemy ; though Void's DPS is nowhere as impressive as Lycan or Ursa's one for example, he generally doesn't need more than 5 seconds to terminate anyone.
The point of low-moderate HP regen in the game, like Vanguard's one, is to allow you to push or farm undefinitely when there's limited harassment and/or you take an occasional hit from the creeps/towers. It also allows you to be at full HP when you suddendly have to take part at a teambattle.
If you're not running short of item slots and you went the PMS/Point Booster way, a Ring of Health in your inventory will be of good use if you need it to stay in lane/keep pushing ; you'll probably upgrade it later into Battlefury anyway...