To quote from the Vegas Licence page re mp3:
"MPEG Layer-3 audio coding technology licensed from Fraunhofer IIS and Thomson.
Supply of this product does not convey a license nor imply any right to distribute content created with this product in revenue generating broadcast systems (terrestrial, satellite, cable and/or other distribution channels), streaming applications (via internet, intranets and/or other networks), other content distribution systems (pay-audio or audio on demand applications and the like) or on physical media (compact discs, digital versatile discs, semiconductor chips, hard drives, memory cards and the like). An independent license for such use is required. "
The last section regarding physical media is a worry. This would also explain perhaps why DVDA doesn't support mp3?
Bob.