Subscriptions in vbulletin do not carry over. People will be able to purchase new subscriptions. Also: The IPN messages from paypal will bounce because XenForo has a different url and format. If enough IPN messages bounce then PayPal will disable IPN which means that XenForo account upgrades will stop to work. Once that happens people are still able to purchase, but it will not cause a member group upgrade.
As you have yearly upgrades it seems to me that you can manage it manually for a year or so until the old vb subscriptions are gone and only new ones are in xenforo.