Here's two potential solutions, someone might have a better one. You could...
1) have a script that runs from init.sqs after the delay you want (say ~30 or whatever)
so in init.sqs
~30
[] exec "createtank.sqs"
and then in the script create the vehicle at the location
2) have the tank on an island to teh side of the map and then setpos it to the location you want. you could leave an empty game logic or pallet there and setpos the tank on top of it