I can no longer view an automatic preview of an m2t clip when selected inside the Vegas Pro explorer - I've never run into this before - I recently switched to Vista 64 Ultimate and captured the clips with HDVSplit.
Right. If this is version 8c you're talking about here, do what Marco suggests. But if you're using version 8.1 on 64-bit, then that isn't the problem--because 8.1 doesn't have the preview video function in the Trimmer.
But in using both of these versions side-by-side I got burned by this issue myself, and simply forgot about the preview feature in 8c. I zip back & forth between the two so quickly sometimes that I just forgot about it and then was, shall we say...perplexed...when the behavior was different. I'd take a close look at the different buttons in each version's Trimmer window, if you haven't already.