You have several issues here, all mixed togeather. Let me break them out...
Bad IP Address on Vista.
This is a known issue for lots of people...
"Windows Vista cannot obtain an IP address from certain routers or from
certain non-Microsoft DHCP servers"
Passthru MAC does not work.
You MUST first do web traffic before it opens. Not e-mail. Not DNS
lookup. Not VPN. Not p2p... It is kind of a pre-approval, but you
still must log in without actually seeing the login. Allowed IPs bypass
Captive portal does not work.
This can be MAC address rewrites by cheap APs or it can be some type of
Link Checker not allowing you to go to the login page until it verifies
that it is not spam, which it can not do until you log in...
Each issue is worthy of it's own thread, and has spawned several.