Which one should power up between Vileplume or Jolteon

Vileplume with that moveset will definitely perform better against water types due to the overall weakness amongst electric types. The only reason I could see powering up the Jolteon would be if Gyaradoses are very common in gyms and if you maybe just want to add some diversity to your team.

I think the Venusaur alone is enough, but if you must choose one then go with the Jolteon.

Venusaur is pretty much unilaterally better than Vileplume in this case. Jolteon however may do better than Venusaur against a few specific defenders (Gyarados, Slowbro/Golduck/Starmie with Psychic moves).

Now, if you find you're going up against gyms with 6+ Water defenders (like all Vaporeons) then the Vileplume is probably better than the Jolteon... In that case you probably want both though.

I would only fully power the Venusaur. And if you need two, power the Vileplume. Jolteon does better than vileplume only against Gyarados whick is easy to kill anyway.

I would only power up a Jolteon with Thunder. I have a 100% one that does really well against Gyarados and any other water pokemon that resists grass attacks (or with moves that are super effective against grass or poison). But to kill pure water pokemon, Venusaur, Vileplume and Victreebel are better choices.