What happens when you put vortex generators on both sides of a symmetrical vertical tail?
Just because VGs can delay the flow separation (with no guarantee of success), it doesn't mean you have increased your tail volume. In fact, you have the exact same tail volume unless your vertical tail is separating at small sideslip, which probably indicates bad design or bad manufacturing.

If the VGs do increase the max Cl, it means you may be able get to a higher slip angle, perhaps more crosswind capability if your rudder is up to the task.