I have Vegas Pro 13. My documentary was accepted for a film festival and I created a Blu-ray with 5.1 Surround Sound and had a DCP created with the same surround mix. Now it's been accepted to a second festival and they are saying "MPEG4 is required for all films and 1 back up disc is required as well."
Is an MP4 with 5.1 surround audio even a thing? Or is the implication that they are looking for a stereo downmix? I don't see a way to create an MP4 with 5.1 in Vegas.
Sony AVC render formats offer MP4 wrapping with AAC 5.1 surround audio.
FWIW, I've heard and heard of multichannel audio nightmares at film festivals, improperly calibrated systems, systems with missing channels and/or systems that do not down-mix properly for instance, attenuating or eliminating the center (dialog) channel. I would think twice about submitting a multichannel mix unless you can test it beforehand. For unknown venues, a stereo mix is safer, especially if the audio is AAC.