Debbie Llayne, you are right, none of the muscles in the vagina is officially a sphincter in and of itself.
However, combining a few muscles will have a similar effect of pretty much “closing” the vagina. The most active one to achieve this is the Levator Ani and specifically the PC (pubococcygeus).
I do not have any references as I am not aware of any studies that were done in this area.
I studied with two different teachers that teach this knowledge, and I know a few women who practice it (including yours truly).
However, if you are not convinced — that’s totally fine. The entire point of this article is to inspire those who want to be empowered.