For most things, leave it alone (i.e., smart resample). When you slow down video, Vegas resamples in order to create intermediate frames. If you don't like the somewhat soft quality this imparts, you can disable resample.
In the older versions of Vegas (3.0 and before), you had to force or disable resample, but the later versions are pretty smart (hence the name) about when to resample or not to resample.