Boy do I know about travel vouchers, sorry to say.
We had a HORRIBLE experience with America West last July, when Dh and I took my parents to Vegas. We ended up sitting at the airport for almost 8 hours. Subsequently we missed our show and dinner ressies, and ended up checking into our room at 1 a.m. At any rate, we did get four $100 vouchers. We booked another trip to Vegas in January and used these vouchers and were able to go thru our
travel agent without any problem. She just had to have them on hand when she made the ressies. But then once again, we sat in the airport for 8 hours, except this time customer service at America West could care less. So needless to say, we will never use them again, and I can hardly recommend them.
You had mentioned that online sites get you better prices than a travel agent. What I have always done in the past is use Expedia and Travelocity to get their best rates for the times and dates I want to travel. If for instance, Delta has the best rate, then I go directly to Delta's site, and their online site usually beats it. If you have to use a travel agent, then I just give her the flight numbers, times and dates, and she books it for the rate I have found, if she can't find it for less.
Happy travelling.