In my opinion, Vienna side reins are very, very good training devices. I don't normally approve of gadgetry, but side reins in general are difficult to abuse, mild, and produce correct results in terms of long-term training (unlike, for instance, draw reins). I have a TB gelding that I event, and lunging in side reins has really helped him to lift and strengthen his back. I assume your mare lunges? Here's what I would do: start lunging at a relaxed trot and canter. When she is a little warmer, put the side reins on. (Try to have a trainer make sure they are adjusted correctly for her.) Again, work at a relaxed yet forward trot and canter, this time with lots of transitions thrown in. Lunging, especially in side reins, is to warm-up quietly until she becomes the horse that you want to ride. In other words, don't be afraid to correct and make her go more forward, bend more around the circle, etc. And I agree with the previous poster: although side reins are the "perfect hands," don't substitute them for other training and they should be used as a supplement to good lessons with a dressage trainer. (I'm sure you already knew that, but I felt I had to say it anyway -- sorry to insult your intelligence!) Good luck!
"Horses are self-portraits of their riders. Autograph your work with pride!"