If I remember correctly, I'm not even sure if you can use a Sprint iPhone on Virgin Mobile either. You can only use a VM iPhone on their network.
Just looked at Virgin's website, and it seems to me like they're just clearing out inventory until they can launch the iPhone 6 and 6 Plus. All they have available is the 4s, and the options for the 5c and 5s show discontinued configurations.
Virgin's plan options and discount device pricing looked attractive when I was shopping around. But, the device lock-in policies and spotty network coverage around where I live and work nixed Virgin Mobile from consideration.