High IV Venusaur – moveset really useless?

Transfer. You will have 3 candies.

That Venusaur you evolved is still good against Vaporeons. But Sludge Bomb is a really bad charge move against Rhydon so I wouldn't advise using your Venusaur on that match-up. But you probably have enough vapes to teatteri through Rhydon so your Venusaur is completely usable.

It's still a Venusaur and venusaur is a good Mon, even with that moveset. Unless you get 4 or 5 months more venusaurs, I would keep that one. It will only be good for water or fairy; but water is quite popular.

I evolved the same moveset with my vinewhip (obviously my razor leaf one got SoB...). Against Vapes he does fine, and whilst not ideal against Lapras he's tanky enough to survive a few hits whilst doing reasonable damage in a gym where you need to wring everything you can out of your attackers.

OK, this is where it's really important to not just rank movesets from 1-6 and only go for #1 or #2...but to look at the % of DPS that the moveset will give compared to the best.

In this case, here are the relative DPS amounts for the 3 charge moves with VW quick move:
Solar - 100%
Petal - 97%
Sludge - 96%

Now granted this is vs. a neutral attacker, so you'll also lose some DPS b/c attacks won't be super effective...but still. I mean...if you need more Grass mons on your roster to deal with Vapes, you'd be crazy to transfer him.

No, he won't be as good vs. Rhydons but I'm pretty sure you have Vapes you could put to use there.

Venusaur is crazy underrated partially b/c of how useful he is in the current meta, but also partially b/c of how good all his moves are...so you don't need to evolve 20 to get a good one. On offense, his #1 (VW/Solar) vs. #6 (Razor/Sludge) ranked movesets are less than 10% difference in DPS

To put that in perspective, someone like Slowbro has one good offensive moveset (WG/Psy) and you lose 15% DPS just moving down to his #2. Same goes for mons like Alakazam, Parasect, etc.