I don’t know about image sequences since I’ve never used that feature, but I can say that Vegas has supported tif files for several years. My best recollection is at least as far back as version 6.
I started using PNGs because, during the course of a project using lots of still images, Vegas was taking a very long time to reestablish its window after I had jumped to a different window such as Internet Explorer and was returning to Vegas. Someone told me that Vegas was calling Quick Time to reopen each tif image. I don’t know if that was true, but it prompted me to switch to PNGs.