import maya.cmds as mc
import maya.OpenMaya as OpenMaya
import time
start = time.clock()
def get_mobject( node):
    selectionList = OpenMaya.MSelectionList()
    selectionList.add(node)
    oNode = OpenMaya.MObject()
    selectionList.getDependNode(0, oNode)
    return oNode
sl= mc.ls(sl=1)#selected()
slShape = mc.listRelatives(sl,s=True,ni=True)[0]
# <-- I grab the MObject of this shape in order to play with the API
cubeShapeApi_Obj = get_mobject(slShape)
convertedAlphaColor = OpenMaya.MColor()
vertexColorList = OpenMaya.MColorArray()
# <-- access and play with mesh data with MFnMesh here I choose to initialize it with the previous MObject, you can also do it with MDagPath...
meshFn = OpenMaya.MFnMesh(cubeShapeApi_Obj)
# <-- grab vertex color in one pass  
meshFn.getVertexColors(vertexColorList)
lenVertexList = vertexColorList.length()
sumAlpha = 0.0
# -- Dirty way to do  a vertexID list
# -- in python you can do it with loop but hey why not use the API?  
fnComponent = OpenMaya.MFnSingleIndexedComponent()
fullComponent = fnComponent.create( OpenMaya.MFn.kMeshVertComponent )
# <--  fnComponent.setCompleteData create a full list of vertex ID like [0,1,2,N]
fnComponent.setCompleteData( lenVertexList );
vertexIndexList = OpenMaya.MIntArray()
# <-- store these ID into vertexIndexList  
fnComponent.getElements(vertexIndexList) 
for k in range(lenVertexList):
    sumAlpha = (vertexColorList[k].r+vertexColorList[k].g+vertexColorList[k].b)/3.0
    if sumAlpha < 0.0:
        sumAlpha = 0.0
    convertedAlphaColor.a =  sumAlpha
    vertexColorList.set(convertedAlphaColor,k) 
meshFn.setVertexColors(vertexColorList,vertexIndexList,None)
elapsed = (time.clock() - start)
print "done, took:", elapsed