I also took my tank out of the Garage and then my game crashed. When I started playing again my tank was forever lost. Any way to prevent that? Make high value Vehicles like Tanks not disappear or not disappear the last used vehicles?

When a vehicle has loot and is garaged it gets put into the arsenal.

Its generally ideal to keep vehicles in storage though helicopters need an airfield to be placed again so if you plan to keep using it and don't have one probably just keep it out, and if it despawns you should be able to open zeus if you add a new binding to the zeus open button (assuming you are the host/commander)

Personally i have my Y menu set to Control+Y that way my zeus muscle memory doesnt have to be reworked. as for the crash when removing tanks *shrugs* test it some might be a one off might be that tank.

That being said, vehicles are supposed to persist when they are within your faction HQs area. i'd guess probably no more than a few dozen meters of the flag assuming it wasnt moved. For redundant safety i'd say keep it near the garage box.