......i don't know of a greysheet for VAM-S but this is a good site for them. http://www.vamworld.com/ here is another. http://vamquest.com/
Join the SSDC, the produce the quarterly vamview price guide.....I think the prices in the guide are a little high but it is a retail guide.
Be very careful with VAM valuation.... IMO, most people who collect VAMs like to cherrypick them. As a result, buying prices tend to be on the low side for attributed specimens, and I would be even more cautious than usual when it comes time to try and quantify their value using price guides. Respectfully...Mike