I've done it several times. To crack the coconut I will first poke out two of the "eyes" so I can drain the liquid into a glass. Then I use a hammer and flat-head screwdriver to crack it open. If you're lucky, the shell will separate from the meat easily. But as is usually the case, it breaks up into large chunks from which you'll have to scoop the meat off of the shell.
I use a potato peeler to remove all of the remaining brown skin, but a sharp paring knife will do. Then I add 1.5 cups of water to the blender, and all of the coconut. Start out slow, and work your way up to the high setting. Be prepared to keep adding water until you get a milkshake-like consistency. You may have to use the depresser that comes with the vitamix in order to push the chunks of coconut into the blades.
Have a large square of cheesecloth handy. You'll have to strain a little bit at a time because it's very thick, and squeeze out the liquid by hand each time. Then you can either save or dump the pulverized bits of coconut meat (I've often wondered what to do with it--could I make coconut flour out of it?). Rinse out the cheesecloth after you've dumped the coconut solids, and repeat.
I keep my fresh-made coconut milk in one of those hand-held shaker cups (has the little mixer ball inside). The coconut will separate from the water and you'll have to give it a quick mix before you use it. It can last up to two weeks if kept refrigerated.
EDIT: Also wanted to add, BE CAREFUL when removing the shell from the meat. I sliced myself a good one being careless with the knife. I had lots of bloodied coconut I had to rinse off before I could do anything with it! And don't worry, I was the only one drinking that batch of milk ;)