How can I repair this bathroom vanity door?

It looks like the finish is lifting, maybe from moisture. It looks like a printed finish, so how you fix it depends on what is under it and how you want to look. You could remove the damaged film and either sand it, prime and then paint OR you could buy a new film or contact paper and attempt to get the same look. Particle board is often under vinyl type finishes, so you would want to use oil based sealer and paint for the nicest finish if you paint. Having said that, I have used Kilz sealer and then a latex paint and had good results. It won't hold up unless you seal it with a clear sealer. There are hundreds of options out there if you apply a new film. If it has drawers or another door, you might want to refinish the entire front to get the best look.

Did you already try to fix it Ed by applying some form of wood filler? If that's the case you can try sanding it carefully to keep the same edges. It looks like a very simple door that you could remake out of wood. If you have access to a router that can give you the same rounded edges. You could keep the old door(s) to match stain. Make new ones, stain and poly, use the same hinges or buy new--those are very common hinges and you will find them at any home improvement store. Good Luck!

I would clean it off, use a bonding primer and try to paint it.

It appears that your door is made of MDF (med. density fiberboard). It can be sanded and painted, but I doubt that it will ever match the printed woodgrain. Also because the fibers have been exposed to water they have expanded and you can get it to look good but it doesn't sand to a truly smooth surface. Also because it doesn't respond well to water, I would use an oil based primer to seal it before you paint.

Due to the fact that the door seems to be disintegrating at the bottom, unless you have a LOT of time, I'd replace the door and its neighbor. Since you will never match the print, consider priming your new doors and finishing them with something that goes with the decor in the room. If there are more doors/drawers, you'll probably need to paint those, too. It could end up being a whole room refresh, depending on the color you choose. If you don't like paint, there is always Contac paper.

Habitat for Humanity has doors for vanities, and they are cheap. I replaced mine with doors from there. However if you can sand it down even, you can then paint the doors and the vanity.

Agree with others that have said due to the deteriorated condition of the door, sinking money and time into trying to 'restore' it will not be cost effective. For longevity, durability, and attractiveness, replace with a purchased newer door.

That's a plastic woodgrain film over particle board. No way to repair. You can buy particle board panels usually sold as shelving at Menard's and make your own door. I have used them to make bookcases, cabinets and shelving.

I would just replace both doors, they are MDF and just not holding up for their purpose.