The main point here, as I was not aware Virgin was a CDMA carrier, is: the Sim Card will work for whatever it is used. i.e for CDMA carriers the sim card is usually used for the LTE networks.
If Virgin still uses CDMA for the other networks i.e 3G, then that will not work, as Virgin would need to activate that portion of the iPhone, and they may choose to not do so.
What happens is service will work with the Sim card as long as you are on an area for LTE. If you leave the LTE area, then the Phone needs to move down to another network, and without Virgin activating it, it will not be able to do so.
Also of note, is Virgin may only use the Sim card for Data, and not Voice and Messaging, so without activation, you would not be able to make calls or send messages.
If you really want to test this, you do have 14 days after the date of purchase of an iPhone to return it no questions asked for a full refund, so you may want to try it out and see.
Otherwise, yes, you would need to buy the iPhone from Virgin to make 100% sure everything will work.