Vented loops are mainly there to prevent the boat from flooding if the toilet is located below the waterline, and the loop must be located above the highest waterline (important on a sailboat). If the toilet is already above the waterline you don't need one on the fill side (I don't). If the toilet is above the holding tank as well you may not need one on the discharge side either it but it probably helps keep odors under control. The height of the loop may affect the amount of water that remains in the bowl after flushing. Mine is only a few inches above the toilet discharge (still below the rim) and works just fine, thought I've considered raising it a couple inches. It's nearly a 3' drop to the holding tank.
Note: my diaphragm pump is located below the waterline, so it always gets a prime.