That item with the blue dot is the fuel pressure regulator. Inside it there is a "rubber" diaphragm that helps in the regulators operation. If the diaphragm is damaged then fuel can escape out that line you are questioning. This line is not a vacuum line, it is a fuel escape line. It plugs into the rubber hose which runs between the valve cover and the air cleaner cover - item 1 in the image below. Note that the image shows the rear of the interconnecting hose. If you check the other side you will find an inlet into which your mysterious hose fits into.

Item 1 in this image.