I tried to import a TIFF file into Vegas but to no avail... Are these files compatible with Vegas?

Yes, you can use Tiff files within Vegas but you need to have a current version of Apple Quicktime loaded on your system to do so.

-Dan

Using Quicktime has also always required the "Authoring Components" as well. Because of Quicktime, TIFFs are very slow to use. I recommend changing them to some other format (PNG, PSD, etc...) to simplify things.

Do yourself a favour and convert the TIFF to PNG. It's the preferred file format for Vegas and your timeline behaviour will be much better.