I would say Vikavolt is better. While you can get Jolteon earlier, Vikavolt's STAB moves can effectively deal with Kahili, and if you teach it Energy Ball, it can also deal with Olivia nicely. Vikavolt is a bit sturdier than Jolteon, which is always a good thing, but its Speed is much worse.
So I would say use Jolteon until you can get a Vikavolt, but either works very well ingame.
Hope I helped!