Try CIDR notation, eg 1.2.3.4/32?
That said, it does seem a little odd that you can’t have multiple simultaneous connections with your vpn provider. And those Allowed IPs seem pretty restrictive. What are overall trying to achieve? Maybe the issue is more with your firewall and outbound NAT rules
OK, then you should leave Allowed IPs as 0.0.0.0/0, and simply set up the firewall rules and outbound NAT for each VLAN to use the relevant gateway. There was a topic recently where someone did essentially the same thing - I will dig it out
As an aside, I get the sense that what “Allowed IPs” means may be confusing you. Allowed IPs are not the IPs that are permitted on the local side to access the endpoint through the tunnel. Rather, they are the IPs that able to be accessed through the tunnel via the endpoint, by whatever IPs on the local side are otherwise configured to use the tunnel by routes/firewall rules. Think of it as - “what IPs do I want to reach through the tunnel?”
This is it: Multiple Wireguard VPN Clients https://r.tapatalk.com/shareLink/topic?share_fid=197904&share_tid=20494&url=https%3A%2F%2Fforum%2Eopnsense%2Eorg%2Findex%2Ephp%3Ftopic%3D20494&share_type=t&link_source=appAgain, the key is configuring the firewall rules and outbound NAT so that they are specific to the particular VLAN you want to use the relevant interface and gateway. It’s really an expansion of the idea of configuring a specific IP to use the VPN (which is discussed by me in a topic linked in the above topic) - instead of a single IP, you are wanting a single subnet
