The third wire going to the switch is to power an LED inside the switch that operates independently of the button presses, so I need three wires. The resistor to the VSS is already directly on the board.
Here is a pic of my project: http://i.imgur.com/aV1n6Nf.jpg
It's a little hard to see, but this is a 14x10x22 "box", with a 5x5" panel coming out of the front with my button.
Short description is it's a photo booth. Big red button is lit up, display shows a live mirrored preview from the webcam. Press the button and the button LED goes off, and the green LED comes on and blinks faster and faster until it stops and the SLR captures a picture. Repeats 4x, then the blue LED comes on to indicate that the photos are being printed. After the printer reports that it's idle, the loop ends and the button lights up again, ready for another press.
I am using an HDMI to VGA adapter for my display, and the webcam, SLR, and printer are connected via USB.
The white wires have a little bit of slack I could take out, and they are definitely heavier gauge than absolutely necessary - I just didn't have any smaller gauge wire around.
You can see the enclosure is made from welded steel rod, and it's sitting on a chest freezer in a garage around cars. Not an ideal location from an EMI perspective, I know, but I don't want to have to worry about the amount of EMI in any environment I want to use this in. This will ultimately be enclosed. The toggle switch is used to turn printing on and off. So if I want to troubleshoot everything without using up paper and ink, I turn the toggle off.
As for how often the switch operates, in actual use at an event (my upcoming wedding reception in 3 weeks), I expect it to be idle for no more than maybe 20 minutes tops between "uses".
Thanks again to everyone for your help! Should I try using Ethernet cable instead of these big wires? My understanding is that each wire pair in an Ethernet cable is shielded - and the wires are much smaller gauge.