From my understanding the biggest difference in exterior poly is it has UV inhalators. I have built several bathroom vanities and used wipe on oil based poly some are about 30 yrs old. One is showing some wear*, as I remember it only has 2 very thin coats, I would recommend no less than 3 coats. *FYI that vanity has survived our 5 children
Interior varnishes are fine for bathrooms, even if the area will be subject to direct water splashes every now and then (normal varnishes are actually quite waterproof when applied properly). Exterior varnishes are formulated not just to resist rain but also to withstand a range of exposure conditions that just aren't seen indoors, so no purpose in using them on interior projects.
Note: use more than two coats for areas that might get wet on a regular basis.