Is it possible to host website through VPN by using windows and IIS?
Sure it is - a BVPN is nothing more than a "virtual" network connection, normal routing rules apply. YOu can route any subnet around like you wish.
How do I deal with the problem that more then me might use the same IP of the VPN provider?
You do not. THat is a TOTALLY diferent story - you ask "how can I host a website bhind NAT WIHTOUT (!) beng able to schange the configuzration of the NAT setup. VPN is not relevant here, the same would apply if you would be on the same office connected with Ethernet. ANswer: you can not, you need to be able to set up port forwaring OR have YOUR OWN IP.
THAT SAID: Most VPN Providers do NOT share your IP but give you one tha MAY dynamically change, and it is quite easy to work around it: Take a VPN provider that assigns you a STATIC IP that is ONLY used by you as long as the contract runs. It is not SO hard to use google to find one (don't want to name any here).