Making estimates based on VAM rather than measured power data is quite difficult as it ignores the ebb and flow of a race, whether it was tempo being ridden or attacks, the wind direction, the size of the bunch and so on.
Bit more detail here on some of the problems Vayers evidence has (it is from a couple of years back but i dont think his methodology has changed since then)
http://www.sportsscientists.com/search?q=vayer