晚上好 is used when you call on somebody at night, he open the door and both of you can say 晚上好. Sometimes when you meet somebody you can also use that, only by the time that you've just meet. If you stay with some one till night, you can't use it.

晚安 is good night, used by the time when you say goodbye to your friend at night or your family member is going to sleep, almost the same as the "good night".

                                                                           Chinese Teacher of Tomorrow

                                                                                       Robin.Chen