I used to use deodorant under breast bc of sweat/odor but i started using petroleum jelly instead thinking it was less harmful. read that vaseline/petroleum jelly is linked to cancer/breast cancer more than deodorant. which should i use??

Vaseline will not prevent sweating/perspiration. You may wish to consider different material for bra that would wick the moisture away. Googling the term, "moisture wicking bra" will reveal multiple options. Get HPV, Flu and Covid vaccines. Wish you good health! Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines

Don't use petroleum jelly -- not because of any cancer risk (no credible evidence for that), but because it will make sweat/odor worse. Keep the area clean and dry, and apply a good drying powder, such as cornstarch, after washing.