Instructions
Liberty City Stories
Complete the entire game. This involves completing every story mission, every side mission (including the Avenging Angels missions and SlashTV missions), winning all the races, completing all odd jobs (100 taxi missions, 12 vigilante missions, etc.), completing all 26 stunt jumps, all 100 hidden packages and all 20 rampages.
Go to Fort Staunton, on the northeast portion of Staunton Island. It is the area that you demolish during the game.
Find the ruins of the Staunton Cafe. There will be a large crater next to it with a Rhino tank. You are free to take the tank and it will respawn there regularly.
Vice City Stories
Play through the game until you finish the "Over the Top" mission, in which you must break into the Fort Baxter military base and steal an attack helicopter. The military base will become unlocked after you complete the mission and you will be given a set of Army Fatigues.
Go to any of your safe houses in Vice City and change into your Army Fatigues.
Head back to Fort Baxter and walk through the front doors. The military personnel will not fire on you unless you kill three of them. Grab the Rhino from the corner of the base and drive it out.