VDub should work fine, if it is MPEG2 though you would need VDubMod or similar with MPEG2 support or to frameserve with AVISynth, which leads me to my next point. Subtitling in VDub like all filtering requires converting to RGB, where as AVISynth it does not. If you are using TMPGEnc 2.5.x I guess it converts to RGB24 anyway though, still probably faster to subtitle in YV12 with AVISynth then convert to RGB24. VSFilter/Texsub works with both AVISynth and VDub.