If he lives that close the temptation for him to engage in covert retaliation is high and cost is low.  He can vandalize as long as nobody sees him or records his face during the act.  Criminal damage is low priority for LE and probably zero priority so probably you need to make sure you have full coverage insurance.  He could also take action against you with a weapon so just because you are not afraid of him doesn't really matter.  Bad idea and you should probably just cut ties with both of these two.
 

Keep fuk'ing the shit out of her its FREE!! I wish I was so lucky theirs a couple of them in my hood I would love to fk

Find a way to unwind this affair. Nothing is life is free and the risk are to high.

Train Wreck

Never mess with another man's livelihood, his woman or his automobile; not necessarily in that order  (-:

MfSD>>>>

Getting your rocks off, even for free, isn't worth getting dead. You don't mention whether they are still in a relationship.  If it's over between them does the lady fear him? If so, have her get a restraining order. Then, when you are together and he shows up the cops have a legal reason and duty to get him away.

Keep it going

Oh Hell Yea!  Keep fucking her, he won't do anything stupid if he finds out.  Send us a pic of her sucking cock please!!

As a woman I think if your already worried about what could happen you have a feeling and shouldn't do it.. most people don't worry. Ijs

First of all, I want to preface this comment by saying I wish I were you.  I mean a woman shows up for sex and it is outstanding, and she has been back several times... don't listen to people who are telling you to cut it out.  I say keep it going.  The father doesn't own your lady friend, and he is not the boss of you either.  The father not liking you is a typical situation of any guy with any lady friend anyway.  

But I digress... now for the advice you asked for:

If you are able, have a security camera watching your parked vehicles. If the guy vandalizes them, you have proof and can have him charged with a crime.  Security cameras are also an excellent deterrent.  When I installed them in problem areas in the past, what happened was the problems just went away.  

If the father were ever to physically accost you, I suggest you pepper spray him to neutralize the threat, then call 911 and let him answer to a judge for what he just did