actually the vanities are about 70-71". the overhang from the countertop makes it 72". you could have your countertop cut a little shorter, or flush w/the vanity base.

find a vanity with no countertop and verify the measurement. then have your own countertop cut to fit.

You could also notch out a bit of the drywall if it's tight.

Or, have a custom cabinet built for you

Matt brings up a good suggestion. if you do the 60" vanity, just continue the countertop to span the rest of the distance. use it as a make up area, or do cubby hole/towel storage underneath.

Or go vertical w/the extra space

(measure your 6' vanity base though. I think you'll be surprised. )

It's easier to buy cabinets as components and put them together. I forget the space I had but I have 5 separate cabinets in our vanity area.

How could you tell if it's going to be less than 72 inches though without physically seeing it? a lot of websites don't specify and just say a vanity is 72 inches.

Do you think just using the existing vanity and getting a new counter top, hardware, sinks etc. would be more cost effective, since it's already custom and built for the space?

Whatever you are buying even online should list dimensions and specifications - it wasn't a surprise I knew I was buying a 70.5" wide dress

The best way is to get the cabinets made for the space then order the counter top. By the time you playaround with ordering on line and returning stuff you could just do custom. BTW to go 60” is going to look bad since the spaces on either side are not large enough to look useful so it will always look like you couldn’t afford the right piece.