Unless proven otherwise, I would assume that VDB compensation will be paid as a voucher that can only be used for SAA. It might work also for SAA's regional affiliate and a SAA marketed flight (codeshare) ticketed on SAA ticket stock *might* work, but if it matters, IMO you should assume no regardless of what the GA might tell you orally.

BTW, I'd also be concerned about the potential future value of a SAA voucher if SAA ceases operations due to financial difficulties.

OTOH, in some countries you can get cash or a check/direct deposit/general gift card for a VDB/IDB, so that could be an option that would work for you, although these amounts tend to be lower than what would be paid in the form of an airline voucher.