I have hpv, but why does my vaginas itch when there are no warts?

Multiple reasons. Itching or irritation can be from many different issues. Moisture, shaving irritation, yeast infection, contact irritation from lotions, toilet tissue, pads/tampons, laundry detergent, etc... Can all cause vulvar irritation (the outside part). Try to see if there are any new exposures, visible rash, etc..
Hpv types. There are about 150 subtypes of hpv and only a few cause genital warts. Other forms of hpv can cause cervical precancer (dysplasia) and cancer, others can cause skin tags and still others cause no problems in people. If you have vaginal itching go see your gynecologist to exclude a vaginal infection (vaginitis), which can be due to yeast, bacteria or other non infectious causes.