-one possible idea is because its got kind of the two head thing going on is that it's ice type moves have a higher chance of freezing the target while hail is up. I don't know if doubling it would be enough or raising it by a flat amount, so like either shifting ice beam and blizzard (and other ice moves) to 20% to freeze or giving them a flat boost of like 20/30% to 30/40%. Because that may or may not be enough there could also be an increase in vanilluxe's special moves damage against frozen targets by like 1.2 or something. I like this idea because I feel like the frozen status effect is something that is underutilized -especially in comparison to burn freeze and poison- and it would give vaniluxe a unique niche as a support or status inflictor that ice types don't really have.
-another possible idea is giving the player the option to mix and match two of its three abilities giving it multiple different playstyle options. having weak armor and ice body for example gives it a healing option after taking damage to get faster. I figure this could be kind of a nightmare to program so it may not be viable.
-I'd also second @Balthazar_Fang's idea about having an ice solar power of giving a 1.25 boost to special attack in hail in exchange for 1/8th or 1/16th hp.
Frosmoth crest: Since it has an ability to alleviate some of its defensive troubles already I would also give it a moxie like ability but for speed, or just giving it speed boost as an ability option to give it more viability in its speed tier.
Froslass crest: outside of other ice type form switchers (eiscue and G. darmanitan), weavile, and slush rushers (beartic and A. sandslash) froslass is one of the fastest ice types but that is all it has going for it. in response I would either:
-reinforce that playstyle by giving it a modifier that if it goes first its special type moves are given a damage boost 1.25/1.5 in power allowing it to hit harder.
-buff its role as an evasion mon by again saying that if it is the faster Pokémon maybe it has an extra evasion stage giving it more synergy with snow cloak, while still giving it counters in things that are either really fast or are hyper accurate/can't miss.