i think that for some codecs it does use the quicktime engine, yes. However, quicktime pro is not faster than the non-pro version. Pro version simply adds encoding support, nothing else.
All QuickTime Pro does for you is unlock some additional functionality in the QuickTime Player. A QT Pro license allows you to encode files from within the QT player, and to view more detailed properties of the file you have open in the QT player.
It doesn't do anything extra for using QuickTime from within Vegas.