Is it a bad idea to run a vpn from the same server which is serving a website?

Its not a particularly bad idea, but it is not an ideal. If its an option, a VPN should be run on a router or dedicated box. Its not always an option though...

Things to consider (and these can all be mitigated) would be -

1. A VPN - particularly a VPN client - can modify routes. If not set up correctly this could cause reach-ability issues for the website. (This can be mitigated by ensuring you don't broadcast a default route). Similarly, it could mean you can reach the resource across the VPN but it might not be working to others not on the VPN.

2. Running a VPN is another service - so another thing that can be hacked or compromised. A lightweight, trusted VPN is not that likely to be an attack vector, but still, additional open ports, more code. More log files that can cause issues if it fills a disk.

3. Its unclear why you are running the VPN. If you are routing through it it can slow down access to the website because it shares bandwidth with it.