It would not look good to have the 72" vanity against the wall. It will throw off your mirror and lighting because the sink will be off-centered. I wouldn't want an inch gap with the 80" though. I'd purchase a custom-fabricated counter and use fillers for both sides. If the vanity color is hard to match, you can set the fillers back an inch or so so they disappear in the shadows.

i should have been more clearer. Everything is coming off the,mirrorors,etc. now if i use the 72 inches would i have to center it between the two walls or can i just butt one end up to one wall and leave myself a gap of twelve inches on the other side of the wall for hamper or garbage pail.? Would it look terrible like that? Or should i just center 72 and leave 6 inches from wall on each side?

Yes it will look pretty terrible with 72 shoved against a wall., and may need plumbing moves as well. You're not saving much over a new one, custom from any decent cabinet shop that actually FITS the space. Go to Home Depot or Loews! Fill it in as now.

With 80, you will have no side splash, and can count on drips down the sides, ruined "skins" on both sides, and a spot to collect hair and dust you can't reach. Go to what FITS Or paint the one you HAVE.

Get RTA (Ready to Assemble) assembled cabinets then 2 fillers against wall. It doesn't have to be custom.

Photo please, so Houzzers can assist better ;)

I understood your post. You have an 82" alcove and you want to chop 10" off for the new vanity, which means the sink moves to one side by 10". The mirror and lights need to move with the sink, so they won't be centered in the alcove. Even if you use a smaller mirror only over the sink and sconces, they still won't be centered. You need a counter and cabinets to go the full length. Or you need a 60" vanity.