Throat vibratos - I am assuming that you are able to bend notes without a problem. If not, first master bending. If you're already good at bending, read on -
Performing a throat vibrato is difficult to learn because you have to do something you've never done with your mouth before. This is not a technique that you will learn overnight. It'll take time to develop it. Just remember that everything worth learning comes with a price. In the case of this technique, time is the price that you have to pay. Every harmonica player has to do it, so just be patient. It's really worth it!
To learn this technique, practice imitating the sound of a sheep/machine gun. You don't have to be able to do it really fast. I would recommend practicing this on the 2 or 3 draw of your harp with a metronome. Now, the big secret - To get a vibrato, try bending the notes with your throat (instead of your tongue).
A good way to practice would be to play the 2 or 3 draw, and try bending it without raising your tongue. You will feel a pulsing sensation in your throat. The important thing to remember is that you are trying to get a smooth oscillation of pitch. Your vibratos might sound chunky at first...but you have to work on it till it sounds smooth and natural.
Try playing the 3 draw bent a half step. Then, try bending that note with your throat. You will start to develop a connection between the note and your throat and every motion of your throat will be reflected in the sound. This is what you are striving for. Practice this every day for 5-10 minutes, and you will soon get the hang of it!
More info can be found in my lesson here: