Vicodin? Is this the form she uses?

If so Vicodin was reformulated by Abbott to contain only 300mg of Tylenol. Because of this change, generics cannot be substituted for less cost. The only generic available is the old form which if the doctor writes "hydrocodone with acetaminophen 500mg" she may get the old generic form at the lower cost.

Using the old name Vicodin only will only lead for now to a brand name and a much higher tier and copay.

It looks like there are FDA approved generics listed at the Orange Book at FDA.gov but they may not be available yet?
Norco is another brand, with hydrocodone and lower Tylenol (325mg), and can be written for in the appropriate dose and still get the generic copay.

It is best to discuss with your pharmacist the details, of this change and what exactly that store carries so you can get the generic copay again. It is pretty complicated and confusing.
Once you get the pharmacist advice, then take that to your doctor.

If this is not the specific reason for your copay increase, then it is an insurance thing to help reduce diversion and abuse. Hydrocodone was at one time and may still be the most desired, diverted and stolen narcotic.