hello kormaniac!
i'm vegas user too. i think you got a bug there, a virus of sort. nahhh... maybe its your hard drive, time to defragment probably. if it's too fragmented, then it would't have a good space to render it straight forward. another would be 'effects', if there are too many effects trax with too many in each chain, that would be it. assuming you're mixing down on 15 trax like in my case, 7 trax on drums, 1 on bass, 4 on guitars, 2 on vocals and 1 keyboard.
that's a lot of trax. sure it's nice mixing them down, adding effects & EQ all together playing all at the same time. i do that too. but before i mix them to 1 track, after all effects & all. i create different files on each track (file-'save as', that sort) then, deleating the other trax, leaving only the guitar 1 track(in the case for guitar 1), also the FX chains not used in the guitars. then render a 'guitar 1 final' file. repeat this process for the rest of the other tracks. (though i leave the drum tracks separated). then go back to your 'overall' mixing file, or you may create a new one, this time using the 'final files' that you've pre mixed & rendered. you can mix down from there. sometimes, i pre-mix down different minor tracks all together. but you should do this when you're already 95% sure that you got the sound you wanted for each track. oh and by the way, make sure you don't loose your raw .wav tracks, so ALWAYS save processed tracks to different name. i hope this works you too, tedious, but you'll get the hang of it.
allen