VPNs are allowed in the sense that the global policy does not forbid you from using them. Additionally, it would be hard for anyone to determine that you are using a VPN. However, that does not prevent wikis from making their own policies about VPNs and what the consequences are if a user is suspected of doing so.
What most likely happened to you was you get an IP address that was already blocked on the wiki because someone else used it and committed vandalism or some other policy violation.
When you visit a website, the website knows you not by who you are as a person but by an ID that has been assigned to your device for use while you are connected to a network. This ID is called an IP address and can change when you disconnected and reconnect to the same network. For sure, it changes if you disconnect from a network and then connect to a different network.
When you use a VPN, you are not directly communicating with the website. Instead, you are using a middle man (the VPN) who communicates with the website on your behalf. The IP address that the website receives is whatever IP address the VPN has decided to assign you. Typically, this changes each time you login to the VPN. This is how VPNs help keep you anonymous. Your IP address is always changing so websites don't know you are the same person.
However, there is a limited number of IP addresses so the one you got this time is the one someone else got last time. If any of the previous users who had your current IP address got blocked, then the IP address may have also gotten blocked. Since the wiki doesn't know you aren't the same person, it also blocks you as long as you are using that IP address.