I haven't used Vegan so I cannot help you regarding the ROM itself, but there are several options of what's causing it.
What type of video, SD card speed/class, how big the movie file is etc. can help someone determine if the video has a chance of playing smooth in the first place on the Gtab.
From my reading that should handle most video, but not all video types are actually decoded the same. If you can give that info someone can probably determine with more certainty where the problem lies or if they can verify issues with Vegan like yours.
This gentleman, TEK112, is very knowledgeable and highly respected at XDA. Many benefited from his advises.
Btw, Beasty rom has retained some vegan kernel plus that of NI and Clemsyn (?). This rom played Plantz vs Zombies just fine.
Dave,