Ok stop, lets clear up a few things first. You are wanting to setup a frontend server cluster that access's a backend database? Why is the NLB in a different vlan then the client system, it looks like your needlessly complicating the setup. I'm going to assume your capable of assigning multiple VLAN's to each server.
If this is true then there are a few different methods.
Configure first VLAN 10 for frontend / client connections
Client access's front end servers on VLAN 10. It is up to the servers to access the DB across the back end network VLAN 20. The client shouldn't be able to directly reach anything on the backend network.