Chiropractic adjustments can benefit a horse immensely. My horse sees one as needed.
The chiro will check your horse and watch it track to determine where the horse may need adjustment. Some chiros use a mallet to help get the alignment back, some don't. I've had both done on my horse and didn't notice a difference in my horse from one type of adjustment to another.
When you are palpating your mare's back, make sure you are using enough pressure to get a true reaction and not just irritating your horse. Put one hand lower on her abdomen, almost to the flank. When you hit a truly sore spot or a spot where she is put of alignment, you should feel a muscle spasm.
A general vet may not be trained in chiropractic adjustment. Definitely do your research to find a commendable and efficient chiro.