Vegas Pro 9.0b bug?
 

Try this...
import 1080x1920 30p clip from EX3 into vegas 9.0b using built in capture utility not the 3rd party clip browser. (all worked as it should)

Now take the clip that was imported with 9.0b from the destination folder and try to import into Vegas 8.0c...It will not import.

This is not a bug - native MP4 format is only supported in Vegas since v.9.0. With v8, you need to re-wrap into mxf.