If I am using a VPN with allowed multiple connections can each user/device be able to view traffic of the other user?

If I am using one a VPN Service, but it allows up to lets say 8 devices to connect, are all the devices/users able to view traffic from other users/devices?

That depends on VPN server settings, but usually yes. The connection between a client(s) and server are encrypted, but the subnet in which clients are usually not (but again, it depends on vpn server settings)

Or is each VPN connection creating its own tunnel, thus its basically saying that only 8 Tunnels can be made/created at once?

It depends on the type of VPN you are using (meaning the VPN server settings), but usually, VPN server creates a subnet(s) for a set number of clients, imagine it like a highway with several lines (allowed number of IPs) with the cars (clients) that cannot share the same lane, but can see each other, communicate and so on.

Also once connected to my VPN is it possible for someone to de-authenticate me, using my VPN also, if someone is messing around with a Wifi-Network?

I mean in theory if you can think of something it can be done, but I am wondering if this is an issue that happens often?

Not without knowing your credentials (accountpassword).

The way you are describing, it seems you have been a victim of an impersonation attack (assuming). Since you describe that a group of people were creating a wifi APs with the same name as your wifi AP, it seems they were trying to make to connect to the wrong one. If you did, then getting your credentials is an easy step.

TLDR: No

Long answer: Although you may be using the same credentials for each device, each device will have to set up and maintain a unique tunnel and will have a different IV for each tunnel. From server's point of view, you're simply accepting up to 8 different concurrent connections (IP:Port pairs) for the same identifier, be it an username or a certificate.

In case you're behind a NAT device, like your home router, every device will have it's own RFC1918 IP address. In this case even though the public IP address is the same from the server's point of view, it will distinguish between them by simply looking at their IP:port pair as everyone will be using a different port to communicate to the outside work, courtesy of NAT/PAT translation.

To sum it up: Even if you tried to intercept all traffic, you'll figure out that there's no no way to intercept of infer what's going inside each tunnel. (This is not particular true if the same IV is reused or due to implementation errors or known flaws but that's not the scope of this question.)