Given the vagina is an orifice, I assume you’re referring to spanking the vulva, and in particular the fleshy folds of the labia. Being highly sensitive, a well-placed smack can be both sore and stimulating, but I’d only spank there sparingly, never more than one smack at a time.
Unlike the buttocks, which are sufficiently padded to take a whacking and heal quickly afterwards, the labia are delicate and can bruise quickly. So when I spank between the legs, I prefer to strike from behind, so the smack lands on the inner parts of her cheeks, over her bottom hole and perineum, perhaps even tingling the lowest part of her slit. It can be very effective to tell the miscreant to hold her bottom open for this kind of spanking.
Best to avoid using hard thuddy objects like rulers and hairbrushes when spanking delicate areas like the genitals. Instead, use a few fingers of your hand, or a light flogger or leather-tipped crop. Your intention should always be to create a sting rather than an impact that will bruise.
As for inserting objects into the vagina as punishment, I personally would advise against it. Unless it’s thoroughly (sterilised) clean, foreign objects risk instigating infections. Personally, I believe the best way to punish a vagina is to keep it empty.
Have you considered making a chastity belt?