Should I power them up?

All I can tell you is what I'd do: Probably yes for the Rhydon and definitely no for the Vape.

In my area, Rhyhorns are pretty rare. I've been able to evolve only 5 so far and have never rolled that move set (or any move set with Mud Slap). So I'd be thrilled with yours. But Eevees are everywhere, so it's not worth it to me to pump stardust into any of their evolutions. Just use that one as-is (it will serve you well) until you get lucky to find a high CP, acceptable IV Eevee in the wild. They're not that uncommon (at least for me).

Mostly agree with howler, until I saw the amount of candy you had for rhydon. Maybe roll one more if ivs are that important to you.

I wouldn't personally. I would upgrade rhydon as is. Vape is not worth upgrading, just catch a high cp, as howler stated.

Rhyhorn yes, Vaperon no.

I use a couple of Evee evolutions with weak IVs - one a Flareon and one a Vaperon, but they were wild caught 800+cp Evees with 60%ish IVs that I just evolved. Perfectly usable as attackers with 2500ish cp but not worth investing in further.

I've had a WaterGun/Hydro Vaporeon since early August and well before I knew much about the game.
I powered it up and later found out it's only 6/15/3 53% IV.
However it has always been a very very strong attacker.

I decided to stop powering it up a few levels ago as it's effective as is (2103 CP or something pre-buff, now it's 2362).

Your Vaporeon at 1756 willl still get the job done imo.

I would not power up neither. At level 30 you will evolve 2500+ cp Vapes and 2700+ cp Rhydons. I have a 1200+cp Rhyhorn and a stack of 1100 cp rhyorns. I think for any HIGH ACTIVE PLAYER its best not to power up commons like ryhorns and Eeveelutions until you hit level 28 at least and strongly suggest you consider 29-30. Every time you level up the mons you catch in the wild go up about 10%.
Rather then fighting gyms your time will be better served going to high density nests and grinding XP.