The CSS can't use its circuit ip address for connections hitting a VIP.
However, you can tell the CSS to use a natted address of your internal vlan.
For this, use the followings config.
Assuming your VIP is like this :
owner blah
content WEB
vip address x.x.x.x
add service server1
add service server2
....
!
Then configure the following to achieve natting of client address
group blah
vip address y.y.y.y
add destination service server1
add destination service server2
active
The y.y.y.y address can be whatever address you want but not an ip address already used for a CSS circuit address.
Regards,
Gilles.