Hey, I'm not sure if I'd put cortisone on my vulva without assurance from a doctor that I should. And that is not to kill bacteria. It . treats inflammation Vaseline is probably safe. It's not medicated. I think organicsooshi is right though that you should see a dermatologist and they may recommend corizone or it could be eczema that happens in that area too and then you treat with products for that. Hope it gets better.
Go to your dermatologist for a better look, but for now, put cortisone on the area every day in the morning and at night. It'll burn at first, but that's killing off the awful bacteria trying to infect you.