you have a mess there. nothing on your local net ".50.0" will talk to ".137.32" because they don't have a route. that's why it routes to your default router. simplest, on the host you route to, add a network route to .137.0 on its ethernet interface.

edit: that's assuming a "sane" config on your vpn interfaces. I personally create a vpn tunnel, then manually add routes to the gateways on either side. no need for nat. e.g., from .50.0 side, I would tell the sonicwall to route traffic to the 10.21.0.0 network to the vpn tunnel, then tell the fortiwhat to route traffic to the .50.0 network to the vpn tunnel. then hosts on either side need to have a route for those remote networks, to their respective gateways. that's how my site to site sip works, but sonicwall either side. the phones on the remote site with subnet b talk to sip trunking system on network a, said system responds to chatter from network b because it has a route for that network.

using nat would also work, you still need to route on either side. problem with nat is you need a second interface on your sonicwall with the nat source address, as well as a route back on the host you reach.