I have a vanity light. Power comes into the light. A wire runs to the switch. I am planning on installing a bath fan that has fan/light/nightlight. The existing switch is mounted in a 1 gang box in beadboard. I would like to avoid cutting in a 2 gang box. I also have a 12-2wg romex unused running from the panel to the attic where the fan is going. I have a 1 gang stacked switch for fan/light/nightlight. I would like to just run a switch leg to the same box and switch both lights on this switch. Does the two power sources make this immpossible? Last resort, can I use the same swich with one light/the other light/ fan and eliminate the nightlight?
No to Q1, switching both lights, unless you get the same power that the existing light switches to be in parallel with the fan light. That would mean running the same gauge 12 or 14 -2 (hot, neutral and ground ) to the fan light from the overhead light.
Combining circuits risks having 240 VAC across your fixtures. It can be done, if neutrals and ungrounded (hots) are kept separate (from each other and not connected through a device section to another.
Maybe to question 2, if you mean using the one (original power source) to switch fan, light, fanlight together. (this assumes the added device wattage doesn't exceed the wire capacity.
You would be better served by biting-the-bullet and put in a new switch box and use the separate power available and get all 3 functions separately Am I guessing correctly that the bath outlet is on the same circuit as the light and they are 14 gauge?? One hairdryer-light-fan-fan light will likely overdraw a 15A breaker
There may be a solution, depending on if, where, and how power continues from the vanity light. You might be able to abandon the vanity light power and switch it in parallel with the fan light, using the fan power feed to power everything.
How many conductors run from the vanity light to the switch box, not including ground? Are there any other cables coming into the vanity light? The switchbox? How many conductors each?