I'm a little unclear as to what you're asking. It simply imports the data that is stored in vB as best as it can. If you're doing something with a subscription that XF can't do, then we can't import that data. (Previously, paid subs weren't imported so expirations, etc wouldn't happen at all.)
Recurring subscriptions will always be a problem. Strictly speaking, you would need to cancel these and have people start them back up (or you'd need to manually maintain them).
If you're using a third party subscription system, I can't make any comments as to what would happen or how that would work.