While the lips vibrate, producing a sound, it is my understanding that this is consequential to forming the lips to do so, rather then the result of a deliberate buzzing into the mouthpiece. You can buzz your lips and then add a mouthpiece, but when you do, you may or may not continue making a sound. You can create a sound on a mouthpiece and then remove the mouthpiece from your lips, and when you do, you may or may not continue making a sound. You can play a certain pitch on a mouthpiece, and chances are that that pitch will not translate into the same pitch when you insert the mouthpiece into the horn. Also, if you're making a nice sound on your horn and then remove the mouthpiece, chances are that there will be no sound, other than some air passing through your lips and the mouthpiece. If you change nothing and put the mouthpiece back into the horn, the same pitch and quality will return. It's all about the interaction between the player and the instrument.
There has been a lot of discussion about the production of sound in brass instruments, ranging from technical to ignorant. Curiosity must be satisfied, but the important thing is to produce good-sounding music with the least amount of effort. That is the goal for which we all strive and sometimes agonize over.