I would definitely recommend centering the boxed above the sinks. Most light fixtures these days only have a 4″ back plate on them……I think the days with the long bar type of lights where the wire can pretty much come in anywhere are out….
And with the space being 59″ after drywall is installed is a pretty common problem…..and does make it tough finding vanities that fit.
There are a lot of 60″ vanities that the actual vanity is 59″
I sometimes have to plane down a side of the cabinet to fit in place.
Now if you get a pre-made top with it….this will require cutting it down. This can be a challenge if the walls are not even all the way across. You could always hire a granite or quartz place to custom make a top for you