Make sure you set the properties in DVD to AC3 sound, other wise it will have to re-render

When the clips are dropped in DVD-A NO re-render or anything should be required (apart from making the required files for a DVD), so do a test bit first and make sure it is right

FYI, Vegas will render faster on 2 & 4 cores (not for DVD-A) my 4 core does it at 4 times real speed if no effects

Also if you do re-render an MPEG out and need to adjust a bit, you put it on the time line and re-edit and as long as you render out in exactly the same format no re-rendering will happen, only any effects will be rendered.