Viper- Pub Match All-star

Skill build is bad (max nethertoxin first), item build is bad (vanguard is terrble on ranged heroes), shadow blade should never be purchased as an escape mechanism because it's not, it's a ganking tool.

I'll forgive these mistakes since you listed sniper and drow as common mid lane heroes when they're dog**** mids, so it's clear you play in a low skill bracket and are still learning.

You also mentioned lifestealer as being a problem for viper, the reality is quite the opposite. The slow from viper strike goes through magic immunity, so you ult lifestealer and he'e stuck in the middle of a fight not being able to move while your team kills him.

Never go for both Vanguard and Shadow Blade: either you take a survivability item ( Drum of Endurance, Hood of Defiance and Mekansm work too) or go for the SB ganking build. If you have enough money to afford both a Vanguard and a Shadow Blade in the mid game, then you could've also picked, let's say, SB and Agh's which is better. I still prefer going for survivability items on Viper, though: the more you stay into the fight the more damage you dish out.

I play Viper a lot and I think it's better to start with ring of basilius and tango. Now that there's no cooldown at all for the poison attack, the benefits you'll get from this item is very much worth it.