I don't believe at this time you can, but if you want to know for sure, just try to import one :) Would be a nice feature to put on the list if it doesn't already support it.
No, Vegas can't support the import of a Flash movie. You can use TGA output for the Flash, or uncompressed/32bit format and you'll get a much better experience anyway. I used to use Swish for my titles in video, outputting tga files. Looked awesome.
...but if you're creating your own Flash materials, you can do the equivalent of what you're asking about.
F'rexample... using a prodiuct like SWISHmax, you can create your Flash movie, then use the program's built-in Export capabilities to export the movie in avi format, and bring that into Vegas.
I did some of that just last night, having recently purchased SWISHmax. (It cost something like $70.)
This answer is probably irrelevant if you're talking only about importing Flash movies from another source into Vegas without having a Flash-package of your own.
Ernie