Vaginal boils can be due to
infection, STD or it can be a Bartholin cyst. Apart from the tests
for this also get tested for diabetes and hypothyroidism. It can also be due to allergy to something like undergarment, soaps, lotions, tampons, sanitary pads etc.
You can soak in a tub filled with a few inches of warm water (sitz bath) several times a day for three or four days. This may rupture cyst and which will then drain on its own.