How do I choose a sealer to put on paint?

If you paint with quality paint you don't need a sealer. For a vanity use oil based paint.

Oil based polys will turn yellow over time usually. I would use a water based poly.

Thanks to both of you

Minwax polycrylic in the blue can works great as a sealer and it hasn’t turned yellow on my white dresser that was painted 5 years ago.

No matter what kind of paint you use it still can chip and crack from use, bumps etc. Oil based polyurethane has a yellow tinge to it and yellows over time. Water based polyurethane is clear and does not yellow. An good brand will work but I biased and use Minwax products.