Don't get me wrong, I would totally support a more nuanced civ screen with historical info, but as I understand it, there's just a single bit for each civ right now that determines whether it shows up. Normally, that only gets flipped the first time a caravan, thief, or ambush/siege arrives (hence why goblin/kobold civs can show up at all).
I just think getting a visitor from a civ should flag that civ as "encountered". If seeing a goblin lasher is enough to know who the leader of his civ is, a human bard should be able to provide the same info.
That's a far smaller-scale change (I think) than, say, allowing the civ screen to be gradually updated with historical events (which would require some substantial recoding, I imagine).
I don't think you necessarily get your visitors from nearby civilizations. If you have a nearby human civ and get a visitor from a farther-away human civ, what would your suggestion be?
So, to clarify: if you've already got a trading relationship and civ screen entry with the Nation of Leagues or whatever (human civ A), and then you get a bard visitor from the Alliances of Uniting (human civ B), you should get an identically-descriptive civ screen entry for them. Obviously, the export/import totals would be 0 (unless you could somehow account for scholars/bards bringing codicies/instruments or taking them with them when they leave), or there could be some flavor text, like with goblins and kobolds.
But what if a vampire had a fake background about being from a different civ?Should you gain information from having them in the fortress? Or gain fake information??
I didn't know vampires could fake their civ of origin. That seems unlikely, because there are usually common aspects of appearance within civs (a vampire from a red-haired, dark-skinned civ would have a hard time pretending to be from a yellow-haired, fair-skinned civ, but from the player's perspective that would be hard to notice). I suppose if this is the case, they should provide a civ screen entry for their fake civ, to avoid giving themselves away.
They wouldn't need to give fake information, the only real non-economic info in the civ screen is a list of nobles and leaders; a vampire creating his or her backstory could easily "research" that info.