I could be mistaken, but I'm fairly sure DVDAS 5 came out with version 10. I actually got the upgrade from DVDAS 4.5 mainly to get the Blu-ray authoring, as I don't generally use VMS.
Yes, you may well be right, TOG. I only caught up with it halfway through.
But Vegas MS 10 and Vegas MS 11 interface with the same version 5 of DVD Architect Studio. (Nicely enough, if you upgrade from version 10 to version 11 using the download from the Sony site, it automatically detects that you already have DVD AS 5 and you don't get charged for it even if you buy the bundle!)