GTA III 100% Walkthrough: Mission 08 - Van Heist
Joe request Claude to steal a armored van that will be passing around Chinatown. Since it's bullet proof, we need to ram it until the guards abandoned it. Then take it to the garage on Portland Harbor. After passing the mission, a contact for missions become available in Hepburn Hights. Also you will receive a message saying you can deliver more securicars at the same place to earn some extra cash. I didn't show it because the message appeared much later after recording the video, as well is not a requirement for 100% completion. Though is worth mentioning nevertheless.
The Bulletproof Securicar is also Fireproof, and obtainable as well. Check the video about it at GTA III Special Vehicle Guide.