Since glass and sand are basically the same sort of thing, and about the same hardness, I vote for the bare vac. line method. I would make a filter sock out of screen to catch as much of the glass as possible after first getting the big pieces out by hand.

Tempered glass usually breaks into pieces the size of rock salt, so hopefully you can get it out without too much fuss. Take your time and grid out the area of the pool and do 1 grid at a time.

I think it might be harder to get clean with it drained than going about it full.