If it's above the vanity then down. If it's on the wall like a sconce to the side, then up. A picture would help.

Medicine cabinet over the vanity, no room for sconces. Current light is facing up, installed years ago, but everything I see now is facing down. Thanks.

Following

IMO it is never good for lighting to light the ceiling and not the space where you need it. This is my favorite light for bathrooms it lights up the whole room and creates no shadows I like the halogen one for this a bit better than the LED. Comes in some different sizes too. It is called Oxygen and is available from all kinds of sources.