I wrote this Blender tool in Python to make it possible to edit Vertex Colors without having to leave Edit Mesh mode and switch to Vertex Paint mode.

It works contextually on both Faces and Vertices, and also has features such as:

-work on isolated color channels
-lock RGBA values
-Sample color from Face or Vertex
-Randomize Color
-Save Color to Palette

Can be downloaded as a Python script:
SetVertexColorTool.py

or as a Blender Extension:
SetVertexColorTool.zip