VS config with multiple IIS sites

Why don't you just have abc.xyz.com and abc12.xyz.com DNS addresses resolve to different IP addresses so they hit separate VS on the BIG-IP, therefore allowing you to do the different things you're looking for.

Either that, or you need to have an iRule that selects a different pool and also disables serverside SSL. Something like:

when HTTP_REQUEST { if { [HTTP::host] eq "abc12.xyz.com" } { pool SSL::disable serverside } }

But separating to two separate VS is the easiest thing to do, unless you have a certificate that has the multiple hostnames in it (SAN or wildcard).