Ask USAir's customer service to explain to you their "Contract of Carriage" rules with respect to your vouchers. Below is a link to their website and to the BBB.
USAir Terms of Transportation (click here)
USAir Customer Service Commitment (click here)
BBB for USAir Consumer Complaints (click here)
It appears that US Air doesn't resolve all complaints.
[...Based on BBB files, this company has an unsatisfactory record with the Bureau due to unresolved complaint(s).]
Here is a link to a website that explains your consumer rights.
My Travel Rights (click here)
Here is one snippet from "The Bumping Game" on that website.
On oversold flights, there is a difference between the ticket counter and the departure gate. If the person checking you in says you do not have a seat, ask if it is because you do not have a preassigned seat. If the answer is yes, tell that agent that, according to federal rules, you can only be denied boarding based on the airline's published boarding authority as follows:
AirTran, Aloha, America West, Southwest, US Airways, and Vanguard: First come, first serve at the boarding gate.
There is more info that can educate you on your consumer rights.
Remain polite and professional, but continue to inquire about your rights as a consumer with USAir. My concern is that since you aren't a paying customer and you are traveling on a voucher, that you may not have the same rights. But, then again you may. Check the terms of the voucher.
I'm assuming that you voluntarily gave up your seat on a previous flight. Remind them how you helped them out previously with an overbooking situation. Is this how they repay that act of generosity on your part?
Very simply, be professional, but be vigilant. Keep a log and record the names of people you talk with and what time. Below is some contact information that you may already have.
Good Luck.
Troy
USAir Contact Information
US Airways employees are empowered to address consumer issues appropriately and effectively at the time a concern arises at airports, city ticket offices, and through reservations.
The US Airways Office of Consumer Affairs will respond to written complaints within 45 days from receipt of the complaint. The Office of Consumer Affairs may be contacted via telephone, letter, or fax.
Telephone:
866-523-5333
Available 8:00 a.m. - 7:00 p.m., EST
Monday through Friday
By US Mail:
Ms. Deborah Thompson
Managing Director, Corporate and Consumer Affairs
US Airways
P.O. Box 1501
Winston Salem, NC 27102-1501
Fax:
336-661-8031
Corporate Headquarters
US Airways
2345 Crystal Drive
Arlington, VA 22227
Telephone: 703-872-7000
David N. Siegel
President and CEO