How do we repair vapour barrier after a flood?
Use a Rotozip (or, like I did, a drywall cut bit in a regular rotary tool) with the depth gauge set for 1/2", so that it cuts the drywall but not vapour barrier. Cut about 2" of drywall off so you have access to enough of the vapour barrier to be able to put seam tape on it. 

You may have to (carefully) cut or rip off the backing paper, but that's not a big deal. 



There is a major downside to this method, if they cut exactly 2' up: you'd have to use either full sheets (wasting nearly half) or cut the sheets into 4'x2'2" pieces, still wasting 4'x18" from each sheet. Depending on the dimensions of the walls in the room, the wastage may be a deal-breaker for this method.