After installing the addon in the usual manner you get two new menu options: one in the Weights menu in Weight Paint mode called VertexColorsToWeights and a corresponding one in the Paint menu in Vertex Paint mode.
Weight paint modeIn weight paint mode clicking on
Weights -> VertexColorToWeightwill transfer a color from the active vertex color layer to the active vertex group. If there is no vertex color layer present a will cretae a new one. You have an option to choose which color channel to use as weight or to combine all colors.
Vertex paint modeIn vertex paint mode clicking on
Paint -> WeightToVertexColorwill transfer the weights from the active vertex group to the active vertex color layer. if there is no veretx group present it will create a new one. You have again an option to transfer the weights to a single color channel or to all color channels.