When you place the counter on top if stock, you will have to use one that may have to be custom cut if the gap is that bad. You should not have much more than a 1/8" gap at that corner. If really bad, you may end up having to adjust with the wall, by using filler strips behind the drywall to square it out.
I had a slight V at the corner and about 8" on my wall where my Vanity sits. It is not showing because we used the 4" Splashes on top of the granite top.
This is something that you are going to have to play around with with the cabinet and use Cardboard as your "filler" for the corner, until you are satisfied that you have it at a point that you can put the counter top on and from the normal eye, no one would be the wiser.
I would probably get a local shop that has leftovers from jobs that you can pick one up cheap and have them custom cut it for the variance if you can get it down where it does not look like the cabinet is crooked or at an angle.