Sorry for the late response, I usually don't use the discussion thread much so I didn't notice this - anyways, in case you were still wondering, most mods are multiplayer compatible but requires that everyone use exactly the same version of the mod. Thus, for example, if one player is using an older version of the mod that was manually installed, but you are using the updated Steam subscribed version, it will probably call issues. Please also be aware that sometimes issues may arise when it seems like you're all subscribed to the mod fine but there are still hiccups, likely due to Steam messing up the download.

okay thanks for the response :)

Good morning

It looks like an incredible mod, I have not tried it yet because I wanted to try it with some friends with whom I usually play all my games.
We have checked that we have the same version of the game and the same checksum.
We have all downloaded the mod from the steam Workshop.

But when it comes to join the game, a message appears saying "The host uses another Mods composition".

Yes it is multiplayer compatible. Appears you have a very common issue when playing with mods in MP. Check to make sure all your modlists line up perfectly, even with the same checksum.

For VIET, you can also download the manual version to ensure you're using the same version.

I have tried on a clean install with just this mod and it still does wrong checksum, is it possible there's something wrong with the workshop version?

@MrTechMonkey If you and your friend(s) still can't get the mod to match up for MP, try downloading the manual install version off GitHub and seeing if that works, sometimes that works better. Steam workshop is notoriously wonky with getting mods in MP to work for all Paradox games and it's largely outside our control.