Reflected sunlight, whether via a mirror or the moon itself, has no effect on an undead creature vulnerable to sunlight.

…direct exposure to the light of the sun. Libris Mortis expands on this description, that text on Sunlight Vulnerability saying that this means The Monster Manual on Slaying a Vampire says, 'Exposing any vampire to direct sunlight disorients it: It can take only a single move action or attack action and is destroyed utterly in the next round if it cannot escape' (253). Other answers may provide pathfinder specific advice. Note: This answer is for dnd-3.5e exclusively.