In the book "The Needs of the Many", it says that the Federation was investigating Vulcan's refusal to help the Romulans before the Hobus Star blew. I didn't see anywhere in the book if the investigation was finished.
This would be a great diplomacy series as you try to figure out why the Vulcan's refused to help and maybe reveal that 8472 was behind the refusal. I'm not sure how the Klingons would be able to do this series though because Fed players might throw a fit at seeing Klingons gain access to Sirius Sector.
Do they still have a Embassy on Vulcan? That might be one way to get the Klingon players to Vulcan rather than opening up Sirius Sector to them. If they do still have an Embassy on Vulcan then you could fly to a Courier meeting place in Klingon Space, beam over to the ship, and it drops you off on Vulcan.
Aren't the Klingon players asking for a Diplomacy set up?