Robert;
I have vouchers in my collection going back to 1999 that are holding up well, I have others fron 2000 and 2001 that are fading. I think it is a thermal printing process and how well the printer is set up controls how well the final product will last.
The vouchers are sensitive to heat and sunlight. The vouchers from Terrible's in LV NV have a warning notice "Do not expose to heat or sunlight"
I am collecting the vouchers as much for the back designs as the fronts and the backs do not fade.
Jim