National Electrical Code 2014

Chapter 3 Wiring Methods

Article 300 Wiring Methods and Materials

I. General

300.3 Conductors

(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment-grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

The hot and neutral wires should be in the same raceway to reduce Inductive Coupling. You'll see using the Right-hand rule, that the magnetic fields created by the hot and neutral wires tend to cancel each other out. Thereby reducing the affects of inductive coupling.

I = current flow direction. B = direction of the magnetic field.

The ground wire was required to run in the same raceway, to provide a safe path for electricity to travel in the event of a short. If one of the wires sheathing becomes damaged, it's likely the exposed wire would have come in contact with the ground. The same way an equipment ground provides protection at a consumer, running the ground in the same raceway provides similar protection the entire length of the circuit.

There's no problem with anything you've suggested. You can use a 15 ampere breaker to protect 14 AWG copper conductors, without a problem. If you feel that 15 amperes is not enough for the receptacles, there is no problem installing 12 AWG wire with a 20 ampere breaker. It's completely up to you what you think is best. If the cost difference is not prohibitive, it may be worth installing #12 on a 20A breaker. Especially if you think you might plug in multiple higher load items at the same time (George Foreman and electric mower for example), or may consider adding additional receptacles in the future.

There is also no problem using a junction box to provide power to different devices. As long as the box is properly installed, accessible, sized appropriately, and adequately sealed.

Nothing in electrical code says that lights and receptacles cannot be on the same circuit. It's more of a guideline than a rule. The general idea is that if the vacuum trips the breaker, you don't want to be left standing in the dark.

The only problem I see, is with the GFCI receptacles in your drawing. You will not need two GFCI receptacles. You'll simply feed the second receptacle, from the load side of the GFCI receptacle. When wired this way, the GFCI receptacle provides protection to the downstream receptacle.