Are the app servers also on the internal VLAN? Is the LTM the GW for the db servers? If that's the case it really shouldn't matter what VLAN the db vip is on, as long as the SNAT is enabled. Putting it on the internal VLAN will remove a hop, so that's good, but I must be missing something in the picture because the first config should work as well. It all boils down to making sure there's no asymmetric routing.
So the packet flow should be: 1. client -> vip, 2. vip -> db server, 3. db server -> whatever client address is given to it by LTM (so if SNAT it should be an address on the LTM), 4. vip -> client.
Hope that helps,
Denny