Is there a tag or should I use the mount and blade warband tag ?
Is there many people on this site that play mount and blade warband Viking conquest? Is there a demand for this tag ?
As Frank pointed out, if people ask questions about this game, then we can make a tag for it. There's no way to have a tag without questions, so at this point, there's nothing to really worry about. If someone comes along and doesn't have enough rep to make the tag when they ask the question, they can always ask for it to be added by leaving a comment or the like.
If people come here and ask questions, we'll add the tag, no matter if it is 1 question or 100. :)
As for the rest of your question, I am not sure how many people here play that game, or what the demand is. I suppose we will just have to wait and see. :)