IP Fowarding

Hi Brian- I'm not entirely clear on your scenario, but you shouldn't need an iRule. However, which of these describes what you're trying to do?

1. Have clients still believe they're talking to server on its old IP address on VLAN 2, but that IP address is now a virtual server on the BIG-IP (presumably the 172.20.3.145 address). If that's the case, you just need to build a "Standard" virtual server--not "Forwarding (IP)"--enable SNAT, and have it send traffic to a one-member pool that contains the 172.20.212.46 server.
2. Use BIG-IP as a router. This would assume clients know about the new address of the server, so I don't think this is what you mean. However, if it is, this is when you'd choose "Forwarding (IP)" as the virtual server type. The destination in that case would be either the migrated server's new IP address (Host) or the entire subnet (Network), depending on how specific you want to be. You don't assign a pool to forwarding virtual servers. You'd want to enable SNAT. Clients would have to have a route to the new network that uses the self-IP address of the BIG-IP on VLAN 2 as the gateway.

Let me know if either of those (probably solution 1) works for you.