this as a solution to VPN's... um, it wont work
firstly the OP doesnt realise that he is not the only user of a particular IP address. othr more nefarious people use vpn's to DDoS websites. so before even getting to a log-in page. a website will review an IP and if it appears as a VPN/proxy that usually is linked to DDoSing, then the website wil block the IP even before the user gets to see a log-in page.
so with that said, how is a user to log-in/sign a message if they are IP banned
This is a very good point. Hopefully we can come up with better solutions in the future. There will never be a way to mitigate DDoS attacks, however, I fear.