How to cover area under vanity with PEX exposed?
Get some fireblocking foam and fill the hole

Since the thing on the other side of the wall with the hole in it is your garage, IRC R302.5.3 kicks in, which requires penetrations in the house-garage wall to be sealed in accordance with R302.11, item 4, which in turn requires an approved fireblocking material to be used for the job, albeit not necessarily something fully tested for firestopping (as you'd find in a commercial or institutional building).  Most jurisdictions will accept a garden-variety fireblocking spray foam for this job; these materials form a char layer on their surface that cuts off their innards from further combustion, thus keeping the fire from getting to the other side for a little while.