First, let me say that in 10 years of experience with the CSS, 2 NIC cards as always a pain to setup/implement/troubleshoot for at the end very little advantage.
Then, if you really want to know what is failing in your case, you should probably sniff the traffic to see what's going on.
The fact that the server and the vip are in the same subnet is not a problem.
However, if the client is in the same subnet as the server, this is a big concern because the server response will bypass the css and go directly to the client.
For this kind of setup you need to configure source ip nating with a group.[ check the configuration guide on how to setup a group ].
Note, if you configure src nating, your servers will only see 1 client ip - the one used for nating.
You lose stats about which client comes in.
Regards,
Gilles.