In Pokemon Go, Vaporeon is one of the most powerul Pokemon at gym battles due to its stats, type and moves. Vaporen has higher HP and CP along water-type Pokemon. Its fast move, Water Gun, and charge move, Aqua Tail, have prominent DPS. Additionally, since Vaporen is water-type, same to moves it can use, the power of Water Gun and Aqua Tail get is increased, called Same Type Attack Bonus (STAB).
Exeggutor works nicely because Vaporen's strength is not effective against to it. Since Exeggutor is grass-type Pokemon, Water Gun and Aqua Tail are not effective against to it. Additionally, Exeggutor has higher CP along grass-type Pokemon.
You may want to have Tangela to beat a Vaporen because of its type, moves, and easiness to be powered up. Tangela is currently the only one Pokemon having only grass-type in Pokemon Go. Since It acquire only grass-type moves, which are very effective against to a Vaporen, you do not have to spend much d time for selecting the best one based on acquired moves. Its fast move, Vine Whip has very high DPS. The value of the DPS is the 9th highest in all the fast moves.
Slowbro is water-type Pokemon that Vaporen's fast move, Water Gun, is not effective against to. It can learn Water Gun as a fast move, so the DPS of fast move is same to Vaporen. Additionally, its charge move, Psychic, is very powerful based on DPS.