Is drinking vaginal juice harmful for boys?

Overall no, oral sex is common in all cultures since prehistoric times and it's generally safe. If however the woman has an STD like genital warts it's not safe to perform oral sex. Sometimes men can also get yeast infection in the mouth if the woman has it but this while unpleasant is treatable. If it's very much a concern you should you use a dental dam when performing oral sex on a female.