Retailers are having Black Friday bundles for Vita, so they'll have stock for that. Also, it's still a new system, no matter how low it's sold in the last few months. I'm sure they'll make sure there's buyback/price protection for any shipments they receive, but there should pretty easily be at least 500K shipped over the Oct-Dec period in the U.S. There'll definitely be stock for Vita to be able to cross 1 million this year, so if it fails to, it will be due to consumers saying "not good enough" as opposed to retailers who've said "enough."
If you're looking for actual figures, all we have is the combined PSP/PSV worldwide shipped figures, I think. Price protection allows retailers to take more risk in accepting stock, though, so the big box guys should have plenty available, though.