I have also installed DVDA Pro 5 (Couldn't find a 64bit specific file so assumed there isn't one)

FWIW...
Vegas puts the mpg2 codec files in the ...\FileIO Plug-Ins\mcplug\ folder -- they originally come packaged in the mcplug_n.cab [don't remember if 8 puts the set-up files in the Sony Setup folder under program files or just expands them in the temp folder like 9 & then deletes them when install's done. DVDA does/has the same, though there are fewer files. Installing Vegas 9C in 7 64, both 64 & 32 bit versions, according to Microsoft's WinDiff, with the exception of 1 file [a newer mcplug.dll in 9C 32], there is no difference between those mcplug folders.

At any rate, if the mpg2 option isn't there in your render as format options [MainConcept MPG2], installation screwed up & that might help I imagine in tracking down whether it's a registry prob or if the files never made it to the proper folder on your hdd. Wouldn't guess it's an issue of the codec not being enabled with a trial, as I assume if you're running a trial you would have picked up 9.

Otherwise 7 64 does have some software installation quirks, that you can mostly get around installing & sometimes running as admin. 9C 32 for example had problems dealing with the Wow64 sections [under HKLM] of the registry, & had to run as admin on this rig -- haven't checked whether it's still there in 9C, as I set the shortcut in the start menu that way & haven't bothered trying it without. I've used some fairly old Sonic Foundry software in 7 -- just finally bothered to upgrade Sound Forge for example -- and while I've had to sometimes set the compatibility to XP etc everything's worked fine.