I do not envy you when trying to reduce the number of vertices. I have played with this a little in RDS, trying to reduce what were probably directly converted NURBS mesh. The decimate function usually produces unacceptable results. The only way I found was to morph/move individual points together and to weld them. Then the single point product must be caressed into the proper position. It is an awful task on a large model. Turns your brain to mush.