There's no difference in doing it in a single VLAN, it's just telling you that you are trying to define a pool member on the same IP address as the LTM currently has configured as it's own physical IP, so it's preventing a duplicate IP situation.
In an typical layer-3 implementation the LTM's self-ip (or the floating IP on a redundant pair) should be the server's default gateway. So if you have the LTM self-ip as 10.10.10.11 you can't assign a server to that IP, that needs to be the gateway address and you need 3 additional IP's in the 10.10.10.x range for the servers.
Denny
Nimbostratus
