Whether or not the computer is a member of the Active Directory domain has little to do with how you access it.
If the proposed VPN connection allows for arbitrary traffic from the web server into the protected network I'd be frightened. If the VPN solution lets you specify traffic filters and statefully allows connections from inside the LAN to access the web server ("Don't speak unless spoken to...") it's less frightening but still vastly less compartmentalized than leaving the web server isolated.
If the convenience really justifies the risk then go for it. My guess is that you can find convenient ways to interact with the web server that don't involve giving it any type of direct communication path into the LAN, though... (WebDAV over SSL for file management, SFTP, etc).