Vector3 Add will add a value to the same Variable while Vector3 Operator will do the math and save the result in a different variable.
It sounds like you just need Vector3 Add. Then its basically just Create Object (or Set Position, changing position of a scene Object) and using that position. The only way to really break it is to run the Add too many times or something, maybe even changing the position after it spawns? Not sure, just need to debug it and make sure no other FSMs are interfering.