Male, Was virgin. Had vag sex. Condom slipped off. Inserted once after. Only Reciprocal oral sex after. Std/HIV chance? Supposed virgin girl but idk.

No way to know. You had genital contact with someone of unknown STD Status. Get tested. .
Yes a chance. Any sex whether oral or vaginal without a condom is a risk for sexually transmitted diseases (including HIV). The chance might be lower given only limited exposure however you can not be certain. You can not tell if a partner has an infection by just looking at them or not knowing in detail their sexual history. Get a check up with urine and blood tests to know your HIV status and STD status.