WebBowser , the problem is that any RD Kingdra should really be special or special-based. Physical walls far outnumber special walls in XY UU and it isn't like Umbreon is able to do much to Kingdra anyway. Obviously, you could slap Waterfall on a RD set to stick it to Florges since you've got nothing better against it and switching is just wasting a turn of rain. But still, I don't see Vullaby handling a typical RD Kingdra set.
The problem for all Eviolite users is Knock Off. For example, Vullaby should be able to outright counter Krookodile, but it simply can't due to Knock Off. Virtually every Fighting-type (Shao, Cross, and Champ) all run Knock Off, so Vullaby isn't necessarily safe against them. With Eviolite removed, Vullaby can no longer switch-in at will against Cross or Shao due to the extreme BP of their STAB moves. Champ is a little better, but that confusion rate has a 50/50 shot of screwing you over. Basically, unless an Eviolite user can still wall most of or the entire end of a spectrum AFTER being hit with Knock Off,
it's very hard to justify using over something like Slowbro, Hippowdon, Umbreon, or Florges who can all still handle many attackers on their end of the spectrum.