Really there is no wrong way to do it per say. If I was doing it, I would have bracing that goes continuously from the front of the vanity into the wall to the back of the wall cavity and is secured to the studs behind the wall so the uplift force when someone leans on it is countered. All you can really do is let the G.C. do what he does and if it fails have him come back out and fix it. If you are worried about it you can always have him draw it out for you and look over it. Text or mail with any questions...
http://www.handymanhill.com
http://www.handymanhill.com