.jpg and .tif both being media events, they just need to go onto a video track on Vegas Pro's timeline - presumably on a track just above the two video events to which the transition is to apply. If they contain an alpha channel, you'll probably have to R click the transition events, select Properties and go to the Media tab and select an alpha choice in the Alpha Channel field.

Hopefully somebody on the forum may have experience with the particular transition pack that you have.