I don't know shit about fuck, but it I don't think a shop vac is gonna clear water out of the line. If it were me, I'd cut off water, open the spigot, and also open some other tap further up the line. Then I'd jam a blower nozzle on a compressor into the spigot and blow the water back up the pipe. Maybe wrap some duct tape around the nozzle to get a bit more oomph on it.