Glad to see you’re getting creative with the toolkit, Hinato! Seems like you’ve got a pretty good grasp of things, and as far as I can se there’s just one mistake that prevented you from achieving your desired result. You do not need to add the location of your actors to the location you get from the VectorFieldArray, as you’re essentially adding the location twice, placing the decal twice as far away from 0,0,0 as you want. Delete the “plus” node and connect the Get output directly to Spawn Decal and it should work. The reason you would want to add something to the output of the Get is if your GridManager is placed at a location different from 0,0,0 in which case you have to add the location of the Grid Manager.
However, I cannot help but feel you’re overcomplicating things a bit here. If you have already placed actors snapped to the grid where you want the decals to be, why don’t you simply get the location of those actors and spawn the decals at those locations directly? I see no need to use the Vector To Index Interface or Vector Field Array in this case, unless I’ve missed something.