Is it even worth it to power up anything aside of Dnite, Trex and maybe Snorlax?

While Larvitars aren't common, Dratinis are sure can be found in most water biomes, Snorlax is less common than Larvitar normally.

70% of the gyms around me needs at least 2650 CP to secure the second spot, though I don't have really good IV Pokémon that has enough CP, Vaporeon isn't enough.

However, since you are at a very high level, a Vaporeon above 3000 CP is pretty hard to beat down, and it is still likely to get a decent spot in a gym.

In the current game, though, consider wait for the gym rework, maybe Vaporeon will be the third best defender overall, only behind Blissey and Snorlax.

If it can't get you a spot, then yeah, no point in powering up...

Eventually, all areas will get to this point so be cautious in powering up low max CP mons...At least until said gym rework...

At least Eevee candy is pretty common. Even in your case, a level 38.5 Vaporeon is still 3089 CP and can get a spot even in your area...you just use them in gyms which aren't pushing 3100+ minimum (which are in my area as well...as most others I'd guess).

Just save your Dnites/T-rex for very very high CP gyms which are stagnant and has rampant shaving...

Tough luck playing wherever you live. I have trouble finding a use for 1100+ CP prestigers.

I'm in the suburbs of south Florida and anything under 2900 won't get you in the top 5. You'll need something 2900 or higher or you'll get shaved out. Most gyms around me start at 2700 now. But good luck lasting more than a day with that.

Machamp

If I had a 100% AT Vape I'd max it out. Assuming you have the dust to spare. If not you might want to save your dust for the gym rework. But a 3000CP+ AT Vape would sit good in a gym.

All depends on your situation. Once you hit 37 you could max out your Vape @ 3090CP which seems really good. I'd do it if it were me. But I don't know your situation.

EDIT - also we don't know how long this gym re-work is going to take...

Congrats. 100% Vapoeon will top out at 3,112 at L38. So, it is worth the investment. Plus, it will become your main water attacker, which you will use very frequently, since there are usually Rhydon(s) in gyms.