Vyper isn't that great in the current meta unless you're very good at aiming. She's kind of in between an assassin and a carry, and is great at chasing enemies. That said, she's not as good at assassinating as Haze and can't solo carry games like some other heroes. That said she moves around the map quickly and can eat up a lot of camps, so you should always be farming when moving between lanes.

Vyper is a beast in the mid-game but yeah, she isn't the strongest in the late game. Keep that in mind.

What items are good for her?

Take one of the top 5 builds for her and you'll be fine. Spoilers: Lots of gun and firerate items.