without seeing the respawn script this is my best stab in the dark
most likely problem if you are using a vehicle respawn script that you know works is that you are using an incorrect vehicle class name for your addon when you create the new vehicle
to check what the actual class name is, simply browse through the mission.sqm until you find the entry for the addon vehicle
class Item42
{
position[]={3582.600586,132.367752,9752.236328};
azimut=180.000000;
special="NONE";
id=79;
side="EMPTY";
vehicle="Truck5tReammo";
skill=0.600000;
text="V20";
init="[this, ""Truck5tReammo""] exec ""common\vehrespawn.sqs""";
};
the vehicle class entry has been highlighted in red for ya