The VIP:80 pointing to pool member(s) on port 8080 should work fine as long as you have (destination) port and (destination) address translation enabled on the VIP. If the proxy servers aren't pointing to LTM for their default gateway, then you'd want to enable SNAT on the VIP as well to translate the source address on the server side connections to the LTM floating self IP on the server VLAN.
If that's not working, can you try capturing a tcpdump on all switch ports looking for the VIP and pool member IP addresses? It would make things easier if you temporarily disable all but one pool member.
tcpdump -ni 0.0 host VIP_IP or host POOL_IP_1
Another option would be to host the wpad.dat/proxy.pac file with an iRule. Bhattman added an example to the Codeshare which does this:
http://devcentral.f5.com/wiki/default.aspx/iRules/Proxy_Pacfile_Hosting_without_need_for_Web_servers.html
Aaron