I got all 100
**SPOILERS**
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Yes the secret vehicle is the Hunter attack helicopter. It shoots missiles and has a heavy machine gun, and you can do vigilante missions with it. BUT! It is located at the Air Force base, and they army will open fire with M-16's the second they spot you. I came in on a Maverick (the Sea Sparrow still remains unfound) and bolted to the Hunter. I had full heath and armor and made it out alive with 48 health
I took it back to the mansion and saved my game. Whether or not it stays remains to be seen.
EDIT - Do you get a cash bonus as well? I wasn't paying attention. I know that I just bought the Malibu and my funds were quite low, so I started my package quest to kill time while my other business' generate cash. Next thing I know I have $130,000. Someone want to confrim this suspicion?