Your Venusaur is wonderful as an attacker with that moveset, but a lousy defender. As such, I would only power it up to level 30. It will absolutely tear through any Vaps or Rhydons. You're not going to see much difference beyond that if you're using it for attacking only. The only reason you should ever power beyond 30 is for defenders, which need all CP they can get. As for your Venusaur, I would save the stardust for something else after level 30.
Power up Venusaur?
So, the grass event is finally over, and I managed to get enough candy to evolve my first Meganium and a second Venusaur. I also was lucky enough to catch a Venusaur today. Unfortunately, my Meganium got Earthquake for its charge move, which was disappointing, but the Venusaur I caught was a low IV at around 1700 CP with Vine Whip/Petal Blizzard, perfect for prestiging. The Venusaur I evolved was 89% IV (15 att, 10 def, 15 hp), and picked up Vine Whip/Solar Beam. It's currently at 1812 CP, but I have exactly enough candy to power it up to level 35.5 if I wanted, bringing it to around 2350 CP. My question is, should I do so? I don't have a good high CP grass attacker at all (not counting my Vileplumes and Victreebels which I don't power beyond 30), so this would be my first. I also have enough stardust to spend, and at the moment I don't have many pokemon that I'm focusing on powering up (one Snorlax and one Blissey that I'm walking for the candy, so it takes a long time in between power ups). I would prefer Petal Blizzard over Solar beam due to its faster cooldown and the fact that it's more likely to become a 2 bar move in potential move rebalances.
Answers
I agree that you can keep one as a prestiger but definitely power one up to level 30 as an attacker.
VW is amazing for attack and both Petal Blizzard and Solar Beam are great for attacking. PB is probably better for prestige as you say for faster cool down and lower prestige mons can't take the extra hits a level 30 can.
Keep one for prestige and power up the other to level 30 as an attacker. Don't really need to go higher than that.
Depends on the stardust resources you have. If you have a ton of stardust and can spare, then do so, as the effort on it is never wasted. Its vine whip and solar beam has enough power, and it has fair defense as an attacker.
Drawback is its max cp is less than its similar grass counterpart such as Exeggutor, and psychic attacks is its weakness. If you are short of stardust and have to get a better attacker, you might want to consider other pokemons first.
If it were mine, I'd likely power it up. I currently have only two attackers that I take past level 30, but if I could ever evolve a vw/sb venusaur it would be my other 30+ attacker.
Considering I've sunk 736 candies into evolving various sludge bomb versions of venusaur, I'd gladly put some of that candy toward maxing a vw/sb instead.
I will put him after my first attacker in case my first one died in the middle of attacking a blissey, or anything else. Because then I won't need to wait for a one bar move to charge up before I can fire at least one two-bar charge move quicker. And the good thing about sludge bomb is, it has very low delay.
I have a 100% RZ/Sludge Bomb Venusaur sitting at 2440CP and will keep powering him up. He hits hard on defense and Sludge Bomb is tricky to dodge.
I will always put him in a gym as a defender even if he has low CP. He's a good defender just low CP. I'm hoping for a gym rework where maybe the mons are ranked by what order the trainer puts them into the gym rather than CP.
If he wasn't 100% I likely wouldn't have powered him up. But Sludge Bomb does hit pretty hard.