Sounds like it is either in the wall or the horizontal run, unless it could be leaking around the vent pipe. Did you have a lot of rain recently? The flashing around the vent pipe that likely goes up through the roof above the vanity could be the source. Unless you can isolate it to that area (can you get into the attic to see if any of the insulation is wet, or along the pipe? can you see through to outside?), then you'll have to tear out some drywall (hopefull, since it is easier to patch than plaster). Depends on if the ceiling is textured whether it is easier to do it behind the vanity or in the ceiling.
If you can rule out the flashing through the roof, then it's probably tear out wall or ceiling (or both, if the ceiling gets truely soaked, it will need to be replaced). Note, though, that water can flow a long ways...could be the tub or toilet as well. Is it a walk-in shower? Did one of the kids leave the doors or shower curtain open and spill a bunch of water on the floor?