Does reducing fruit change its composition? I like fresh fruit reductions like cranberry coulis or lingonberry and huckleberry sauce. But I am worried that I'm not merely evaporating the water, but changing the sauce in other ways.

Does simmering a fruit reduction cause anything other than water to evaporate?

Does reduction reduce or destroy the vitamins by heat? A comment by Johnannes_B mentioned this.
Any molecules more volatile than water will be evaporating more than water molecules, less volatile molecules will evaporate less than water at the boiling point. This is the basic principle behind distillation.
The heat will for sure be driving chemical changes.