1.Stats: Should Vileplume get a stat boost, Evolution, Mega Evolution or nothing?
Probably. Vileplume already got a stat boost, but it is insufficient.
An evolution is impossible, and a side-evolution already happened.
A Mega wouldn't save it, but would make it less useless.
2.Should Vileplume get more abilities?
Yes. Vileplume only has only two Abilities. Grassy Surge would be an obvious option.
3.Should Vileplume get more moves?
Absolutely. Vileplume should get more Dark and Fairy moves like Dark Pulse and Draining Kiss.
4. What additional typing Vileplume can have?
Fairy and Dark are the most obvious options.