Hi Thomas,
Which model of switch are you enabling this span?
Anyways you can have the destination port on any vlan depending on what interface you are monitoring. Only problem is that when you are monitoring a VLAN rather than physical interface you need to be aware that "A destination port that belongs to a source VLAN of any SPAN session is excluded from the source list and is not monitored"
Hope that helps.
Regards
Najaf