Which one should I power up for defending gyms?

I'm not a green ppl but from my experience sludge bomb suck.

I'd probably go for the Victreebel but like, that's just my opinion man!

This is like the toughest one of these questions i've ever seen.

Venusaur's got the best stats, but vine whip isn't great for defense. Victreebel is least tanky, but you got good razor leaf/sludge bomb to throw off attacking anti-grass. That Vileplume is just awesome all around, I'd probably pick him to power up and use him as a grass attacker too though. wish i had one just like it, but mine always seem to come with moonblast.

Man...save your dust to top tier mons for now.

I'd save the dust and just evolve another Vaporeon from all those eevees that seem to be everywhere after this latest migration.

None of the above. I wouldn't defend with any grass pokemon (except for Exeggutor); their lowish HP and weakness to fire and ice (both common attacking types) means they're pretty easy to take out. You're better off sticking a high CP Vaporeon in the gym instead.

vine whip isnt good for def.
i try the 3 pokemon , think the best is victreebel razor/sludgeBomb (for def )