She just got two indirect nerfs last patch (CotC and jungle XP.)
Overall, I think she's very strong right now, but not OP or too oppressive.
Late-game, she can both deal a lot of damage and tank a lot, but she really needs some expensive items to get there. To really get that damage power spike, she needs Trinity Force, which is very expensive, and a Vi pouring all of her money into that item isn't building defences and will be very squishy.
Early-game, her ganks are decent but telegraphed, so you can avoid them quite easily if you are careful.
After 6, her ganks are very strong, though she won't kill you alone if she can't hit you with her Q. She can still use her ultimate to lock you down for get ally.
The ultimate has a fairly long cooldown at early ranks, so she can bring strong ganks but not that often.
Late game, she falls off a bit because she's often not really tanky enough to dive into a full team (unless ahead), and while her ultimate gives her guaranteed damage and lockdown, she's also fairly easy to peel off and burst down after she's used her ultimate.