I don't think that VCD supports 5.1 surround. I do think it would support Dolby Pro Logic (2ch/4ch) sound, but since I don't have the DVDA part of Vegas I can't say for sure.
VCD does not support AC3, thats just a limitation of the VCD format. ProLogic may work as that is just encoded into the existing audio stream however given the way the audio is encoded into the mpeg1 stream on a VCD the information that ProLogic needs may get lost.
I think SVCD will support 5.1, but you don't fit too much video onto SVCD due to its higher bit rate. You can usually go outside the standard specs but then its oftenly a case of checking if it works player by player. If its only for your consumption then not a big issue.