Since the drywall, green board, or cement board is covered completely why does it matter. The exposed area would be exposed regardless.
You can read the manufacturers guidelines. Most tub surrounds offer the choice of using basic drywall. I have a little experience with this and I would go drywall if you can - especially if it for a "cheaper" vinyl surround that has any kind of curves or shelves.
What happens with concrete board is that the vinyl feels more hollow with the harder board and it kind of feels cheap(er).
I almost always drywall now. Now if I feel there might be the potential for any kind of water issue I will paint the drywall with a waterproofing membrane like redgard.
Although the material may be covered there is still a potential for water or humidity to come in contact with it. A small leak may go unnoticed until mold or rot are evident. The water resistance is dependent on a seal whether it is chaulk, grout or some other material. Water resistant materials are chosen for their "just in case" or "what if" properties. Think of it in terms of your roofs. Shingles alone may keep the rain out but it is comforting to know that the ice shield or roof felt are backing up the shingles just in case.