Vestibular and vulvar are both from Latin, while phallic is from Greek. If you object to using a Sanskrit word as parallel to the Greek one, how is Latin any better? To satisfy the hobgoblin of consistency, you would need to come up with a Greek word. Strangely, the classical Greek language seems to lack a word for the yoni. When the Greeks talked about it, they used other words metaphorically: either kolpos = ‘inlet’ or kteis = ‘comb’. (“Comb”? 
Huh?) So you could coin your own Greek-based adjectives, either colpic or cteic.
I still go with yonic, as it’s the term that’s already won acceptance from those who care about the subject. A Google search brings up 5,510 hits for “yonic.” The second and third hits right off the search page tell us, and I quote:
Word: “yonic”.
Short Definition: the opposite of phallic.
yonic. Andrew Van Schaack writes: … The feminine counterpart of phallic
is doughnutish. No, just kidding. There really is a word, and it is yonic.