@ar-thomas VTI IPSec is different that a policy IPSec connection. You aren’t making policies for networks rather you are merely creating a gateway over which you can route specific traffic based on static routes and policy based routing. There was a very helpful hangout that was done on VTI IPSec righty after it was included in pfsense. I’ve referred to it a few times over the years myself. It can be found at:
https://www.slideshare.net/NetgateUSA/routed-ipsec-on-pfsense-244-pfsense-hangout-june-2018
As the hangout and the pfsense documentation indicate, you need to be very sure that the settings, Ike type (should be 2), encryption and hash, etc, are exactly the same in both the 7100 and 5100.
Try setting things up from scratch after reviewing the slides and, if you still have issues, please post screenshots of your P1, P2, gateway and static routes for both sides.
Also, any reason you haven’t updated the 7100 to 21.05?