It's not a "series" -- there's only one volume.

Mystique was originally collected as a four-TPB set. Vaughan wrote the first two (#1-6 and #7-13), and Sean McKeever wrote the last two (#14-18 and #19-24).

"Mystique by Brian K. Vaughan" is a one-shot TPB collecting all of his issues, which also encompasses the first two original TPBs.

If they were to do another volume, it'd be a "Mystique by Sean McKeever" one-shot TPB or something like that.

-Jeph!

Whosoever knows completionism burns at the touch of ... THE OMITTER