Client letter from Vendor for H1B extension

I am working for a employer A and through vendor B and then Vendor C working for Client X. I moved recently from India and i had got only 1 year visa approval which is expiring on Sep 30 2014.

Vendor C through whom I am working at client site is XYZ America, subsidiary of one of the top Indian IT MNC.

I want to go for extension in April and keenly looking for extension for 3 years.

Vendor C shall be giving me client letter with 3 years duration possibly. Do I still need the letter from Client X at whose place I am working ?

Does Vendor C client letter work in getting me 3 years extension???

Quick response is highly appreciated.

Thank you.