Really nice job so far Rob. You're helping to inspire me to start my own cabinet project that I've been procrastinating on.
With respect to the Granite cost, not sure what variety of stone you are looking at, but you should be able to find something nice for about half of that cost, based on the size of your vanity. You're best bet, is to talk to a granite and solid surface fabricator, and see if you can't get an off-cut from a larger kitchen job. If they're doing a large job with similar material, often they will have some substantial leftovers (especially when the kitchen has an L-shaped counter section). Let that customer pay for the full sheet (as they are typically priced), and you can negotiate a better deal on the vanity top.
That's the relationship I have with my fabricator, and what I try to do for the smaller jobs - vanities, fireplace hearths, etc...
Looking forward to seeing your finished product.