They can't open up the Metrodome. It's not built for that. It would be impossible to use it with no roof. Either they fix it (which will take months, well into next year) or tear it down.
Even if none of this had ever happened, the Vikings would still be planning a move. They don't want to play in the Metrodome anymore anyway, never mind the roof business. Even if the Metrodome were completely repaired right now, they'd still want out. Partly because the Metrodome is obsolete, but mostly because of the Vikings having a lease that, to be blunt, sucks.
Mark my words, if the Vikings don't get a new stadium, they WILL move. Where they will move TO is up in the air, but they will move.