Which of these is the best grass attacker into multiple Vaporeons?

IMO Venusaur and Exeggutor would be tier 1 and Meganium and Tangela would be tier 2.

exeggutor

I've always had the most success with Venasaur; at 2068 CP, guranteed to take care of two to three Vapes and/or Rhydons... and mine has petal blizzard... with beam, it should be that much more effective...

Just don't forget Sun Flora... I have 1380CP can take down a 2780 Rhydon...

I feel comfortable using venusaur either with SB or PB. Problem is, I don't have double grass exeggutor to test.
Tangela VW/SB is a solid prestiged. These 2 would be my grass choice

I'm assuming by SB on Egg you mean Solarbeam, but I'm gonna post a little love for BS/Seed Bomb (I've not been fortunate enough to get a BS/Solarbeam yet). I know it's considered it's worst moveset, but if you're a dodger like me it's great fun to use. BS powers up SB so quickly you're basically just spamming SB, which is so quick you can pretty much just keep dodging... Like I said, great fun!

That said, I'm very much a fan of Venusaur, it's been my go-to for Vapes for most my PoGo days. I have 3, all with Petal Blizzard, 2 VW, 1 RL. Even at 300-400 CP lower than opposing Vapes, it can easily take a couple down.