SurViper

Don't try to go damage items for viper. Netherdrake's abilities are quite good. What he needs is to survive. If a viper can withstand damage and live in a teamfight he can kill multiple heroes alone. It's best to stack up on defensive items, perhaps a yasha for movement speed. But it's important that you go for stats or strictly speaking HP. If there are burst damage opponents go a blademail; they won't pick on you as easily.

Early game.

The main reason you dont stockpile on damage items is because of viper's second ability; nethertoxin. Upgrade nethertoxin at levels 1 4 5 and 7. The rest of the abilities at your freedom. That's upto you. Go a hurricane pike as this helps not only to escape and reach locations faster the extra range and strength contributes to survival and better harassing early game and secuing kills that would have escaped in mid and late game.

Late game

Pick your items depending on the opponet hero picks and on your team pics. If you have 2 or more stunners go for tanky items more, if you have only one stunner go for a bit of attackspeed along with tanky items. Preferably have a linkens or bkb depending on the opponent heroes. Take the strength and hp options in the talent tree. If they have a silence go manta. You need to go heavy on the first ability.

1 rule to follow with Viper. Never ever try to run with a viper. Firstly cause your movement speed is ♥♥♥♥ and secondly you can cause immense damage by standing and taking out the squishy ones first like a cm or rubick. But that doesn't mean you'll jump in and tower dive. No. Use your brains and focus on teamwork as to how you can use your allies abilities to your benefit.

Defence of the Ancients is all about teamwork and fast thinking. Oh an common sense. Lots of it.