We do not have option weeks here and are not allowed to work through our vacations. I don't personally have a problem with drivers working their vacations as long as there are no displaced drivers who could be working if the drivers were not allowed to do so. Imagine being a cover driver sitting at home while a FT driver, who has already received his vacation check (45 hours here), chooses to work his vacation for whatever reason ($$$, no life outside of UPS, doesn't like spending time w/spouse, etc).
Now, before W jumps my case, I have done this myself in the past, but only as a favor to my on car and I was given the equivalent number of days off as comp. I forget the details but I think a driver had a death in the family and my on car asked if I could help out for a few days and I did. To answer the OP, when this is done the driver does his regular run and the cover driver is bumped, which is why I don't think this is fair and should not be allowed on a regular basis. The cover did work but just covered another area.