It's maybe a bit late now, but when opening a trial version of Vegas Pro, a window pops up inviting you to continue in trial mode or to code in your serial number if you have purchased it. There's no actual need other than personal choice to uninstall the trial version to download the purchased version.

But more to the point, please advise the full name of the dll that is missing - and perhaps upload a screen dump of the error message to the forum.