Do Virgin FC get access to the same miles flights that Delta SkyMiles customers do or is it a smaller subset?
No they don't. VS don't get any where near as many reward seats as DL offer SkyMIles pax, though to be fair that's how all airlines operate. The flip side of this coin is VS also wont offer the same availability of their reward inventory to SkyMIles pax either.
Here's an example comparing DL award availability with VS reward availability on the same route / date:
ATL>FLL Dec 28 2024
DL have far far more availability on that route on virtually every flight they op that day (approx 1 per hour). VS have just 3 First Class seats on the 06:20 flight then nothing until the 16:26 flight.
VS
DL