This is the correct answer.
This might be the calculation you were going for:
var vector_pad_size = Vector2.ONE * pad_size
var left_rect = Rect2(get_node("left").get_position_in_parent() - (vector_pad_size / 2), vector_pad_size)
var right_rect = Rect2(get_node("left").get_position_in_parent() - (vector_pad_size / 2), vector_pad_size)
It doesn't make any sense mathematically to do Vector2(5,5) - 2
but it does make sense to do: Vector2(5,5) - (Vector2(1,1) * 2)
which equals Vector2(3,3)