Unless something has changed in the last few years, Virgin Mobile operates on Sprint's network and last I knew, didn't support roaming. That all may have changed. I haven't looked into Virgin Mobile in a while.
Virgin used to support voice and text roaming, just not data roaming. Just checking their website now, it appears they do have two types of data roaming: Extended data, which appears to mostly be agreements with smaller carriers (rural carriers and likely US Cellular), where roaming is treated as if it is part of the Sprint network; they also have traditional roaming data, which is likely off Verizon. So they do have roaming, though the roaming (not including extended data) will likely be quite limited and at reduced speeds.
I actually think this move is very smart for Sprint. Sprint owns three different prepaid brands, and have had a problem that Virgin Mobile really hasn't had much of a brand image recently. This allows Virgin Mobile to get plenty of publicity -- with the $1 for a year of service and the first prepaid brand to be sold in Apple stores. It differentiates them from Boost Mobile, who will remain the more "generic" prepaid, but that gets attention because of the various Boost Mobile authorized resellers. And then Sprint Prepaid is the one that Sprint can put people on when they fail a credit check, since it is sold in Sprint stores.