The overnight is absolutely inferior to brining, no two ways about it. Getting the spice onto it immediately after rubbing with curing salt is the key to it- the salt will suck water out of the brisket, it mixes with the spices, then gets sucked back into the brisket. The vacuum bag being under pressure helps to squeeze the whole works, too. It's an imperfect solution when someone needs a corned beef quick, but it does a decent job of it. It'll be a sight better then anything mass-produced for a grocery store, I'll guarantee you that. It works better on thinner pieces, but even a thick piece will turn out pretty good after a couple days.
Given my druthers though, a long soaking brine is the way to go.