Blender version 2.65 Testversion 1

The decimate modifier produces inconsistent results when specifying a vertex group in collapse mode, see attached file. When turning down the ratio of the decimate modifer, more vertices than specified in the vertex group are decimated when the ratio modifier nears zero.

This issue makes it harder to create splitted LOD-models that need to have consistent borders, as well as decimating models that have more than one "UV-island", as vertices are merged across those islands, creating visible artifacts.

Astonishingly, as demonstrated with the open cube in the file, the border is preserved better when not specifying a vertex group at all.