Actually it would cause an integer overflow, a buffer overflow is something completely different. Also it is worth noting is that setting the respawn time to max unsigned integer value wouldn't work in this case due to the way LU handles respawn times internally - they're signed integers, and the internal times are stored in milliseconds (so the user input is multiplied by 1000).
Setting the time to 2147483 or lower should work reliably.
Edit: Technically that doesn't keep the vehicles where they are until the server stops (it's only a couple of weeks, if the server is on longer than that some of the vehicles will respawn). If you want to keep the vehicles where they are for a longer time, you can use Vehicle.SpawnPos and Vehicle.SpawnAngle to force the vehicle to respawn at a given location.