@Mithrarin on Twitter
Vaporeon is out of the running due to the way CHRs are structured in the set, Octillery is #191 while Frosmoth is #192.

I’m not 100% sold on Jolteon getting a CHR due to how many regular Pokemon are sandwiched between Frosmoth and Flaafy (5 if you exclude Basics) but Flareon has a really good chance when you compare how many regular Pokemon are wedged between Charizard and Kingdra (2 if you exclude pre-evos).
Well since there's exactly two spaces between Charizard and Kingdra, they could literally be Vaporeon and Flareon haha, you're right it could be rough with Jolteon tho