They will have to clear immigration and customs in the USA (so make sure they have ESTA or a visa!). And they will have to claim and re-check the bags as part of that.
Odds of the bags being sufficiently late but not completely lost are so infinitesimally low that I wouldn't think about it. If the bags really did miss the VS flight to ATL then file the claim and move on. Otherwise they'll almost certainly be just fine.