DVD A Studio 5 online (F1) Help:
“Audio format: …choose a setting to indicate the format whether you want to use PCM Stereo audio, AC-3 Stereo audio, or AC-3 5.1-surround audio.
Only PCM Stereo audio is available for Blu-ray Disc projects.”
DVD A Pro 5.2 and 6.0 online Help:
“Audio format: …choose a setting to indicate the format whether you want to use PCM Stereo audio, AC-3 Stereo audio, or AC-3 5.1-surround audio.”
You can burn from Vegas timeline with your 5.1 PCM to a disc without any menus.
Vegas Pro 13 (and MSP 13) online Help:
“Burning a Blu-ray Disc from the timeline: …Wave64 (PCM) audio for Blu-ray Disc can use any of the following formats:
48 kHz, 16 or 24-bit, stereo or 5.1 surround (available only for 5.1 surround projects)
96 kHz, 16 or 24-bit, stereo or 5.1 surround (available only for 5.1 surround projects).”