+12
As explained here, the number of EVs required to get +1 stat point tends toward 400 divided by your Pokemon's level. So for your level 55 Vanilluxe, we have 400 ÷ 55 = 7.27.
Your Pokemon has 90 EVs, so to figure out how many extra SpA points it gets, divide 90 by 7.27 to get +12 Special Attack.
There are some other factors to be aware of:
Your Pokemon's stats, which factor in its EVs, are recalculated all the time. That means those 90 EVs which are worth +12 points right now will be worth +18 at level 80, +22 at level 100, etc.
Because of how stats work, the number 7.27 we calculated before is only an approximation. In practice, it will usually take +8 EVs to result in +1 stat point, but sometimes only +4, averaging out to 7.27. The magic number is always a multiple of 4.
You can try all of these things yourself in Pokemon Showdown's teambuilder by adding a level 55 Pokemon and dragging one of the EV sliders up to 90.