How to seal p-trap leaking at inlet
It is not sealed with petroleum jelly but a beveled slip washer.  You can try to tighten the nut on the p-trap where the black pvc enters the copper p-trap.  If that does not work, you will need to get a new 1 1/2 beveled slip joint washer.  Your big box home improvement stores carry them as do any plumbing supply houses.



They also make rubber washers for this fitting:


But I prefer the beveled slip washer since it works on compression.