Allowing for what theprof00 said:
1st week: 450k
or 1st week (weird 3G only thing): 100k, then 2nd week: 380k
I think it will roughly start similar to the PSP (which managed 400k apparently), the PSP was an unknown (a non-Nintendo handheld?) but was hyped.... the Vita has the advantage of following the PSP, so normally I would have put the launch higher, but my limited knowledge of people's opinions of it seems to indicate it isn't that desirable.