Get it together, Limey. What even is your life? 
...xP

It was vague, but the war has been going on for a while now. About 100 years (I believe I specified that in the Setting/Plot...but I'm too lazy to go back and look).
A Vasto Lorde is not an Arrancar, it's a type of Menos which is a Hollow. A Vasto Lorde never removed its mask. Harribel was a Vasto Lorde before taking off her mask.
What is Seiretsu? Are you meaning to type Seireitei?
Yasu was not called to fight the VL. It would have been reported to her but she would have only been dispatched if the situation was dire/more Captains died. Since it was the 8th (Information), she was not as equipped in straight combat like another Division (5th). Kuroda was aiming to protect the 8th Captain and return, but that mission failed (but I left that out on purpose). I would have said that Kuroda wanted Yasu to go as well. But Kuroda believed two of the best Captains could at least drive the VL away.

Other information you should know, It's probably going to be a lot busier for Yasu and that she's not going to be able to perform any physical duties herself. The Captains will have a lot of paperwork and organization/training to do during the war. 
The Third Division is more in charge of speaking with the Houses, the Sixth would enact decrees and work to fix injustices rather than investigating it, which is what the 8th would do. So the Third discovers there is a problem which is processed through the Eighth and then enacted through the 6th. (This doesn't change your post, just something to consider)

There might be more, but I can't think of it right now. Sorry for the lack of communication. :(