Ok well I have not tried to tighten this point. This is serviced as a complete unit and as far as I can see there is no way to tighten the mirror inside of the mounting base. As I stated though I have never tried to repair a problem such as yours. It may be worth a shot to remove the mirror and see if there is anything that can be tightened within the actual mirror base. The problem is to even access this point you must remove the door panel to access the internals of the door. here is how to do that.
If you would like, I could opt out so that other experts could try to further assist you. I wish I could be certain of what is in there to tighten up, but I just have never had this problem and there is no procedure to actually do this. As I said if you would like, I can opt out so that other experts can answer. Good luck!!