There shouldn't be any legal restriction.

If the Dolby Digital software is licensed for use with Vegas, as it is, and Sony call Vegas a 'Pro' application they can hardly suggest all you can use it for is home videos.

Dolby however tend to want to have some input on the production of material carrying the Dolby Digital logo. This includes certifying the mixing suites in which Dolby Digital material is produced so I wouldn't be putting a Dolby logo on your work. Other than than I doubt they'll be chasing you for using AC3.

What the proviso is most likely saying is that the AC3 file that is generated by Vegas is not the type used for broadcast. In fact most broadcasters would want unencoded discreet tracks as Dolby Digital encoding is (or should be) a encode once-decode once format (though users of the newer AVCHD format cameras might disagree). The broadcasters then encode the discreet audio with professional Dolby Digital hardware encoders at the transmission stage of the program or ingest onto their video servers. (I won't confuse the issue with the use of Dolby E as a transport format on digital stereo tracks)

I certainly produce corporate videos using Vegas and encode both 5.1 and stereo AC3 tracks for the DVDs that are the end product. I think you would find many others here that do the same.