Vehicles cannot respawn if players are inside them. No object can be destroyed if a player is riding in it, meaning that object_create_anew will not affect a vehicle with a player inside it. Either way, the way to respawn extra vehicles is not by using a timer.

You have to add an extra vehicle spawn point (with the vehicle type set to "none") to the top of the Vehicles list in Sapien. The spawn point on the top of the list is always where vehicles not in the globals would respawn normally if they were just spawned and left alone. Then, you place a trigger volume over this empty spawn point, and check continuously to see if a vehicle is inside it. If a vehicle is detected, then it is re-created to put it back to its actual spawn point. This makes it appear as if that vehicle is being respawned in the same way as any other vehicle that is in the globals.