These days USCIS need client letter and will send you denial notice if you could not provide all the needed documents. You can do one thing - File with prime vendor's client letter for now and in case if you got RFE asking Employer client letter then you can go to employer and ask them to write some letter stating you work as a contractor. You can just ask to client manager(to whom you report in Main Employer office) to write something stating you work under him/her. This should help.
Since your Prime vendor has mentioned your role as consultant that means he is also deploying you to some client location. If your prime vendor could give a client letter mentioning you work for their internal project then you do not need client letter from your employeer. But this is not highly recommended. 
Better talk with your main employeer and ask them to give some letter or something stating you work for them. You do not have to tell them to give client letter...they might not give client letter but will give something in written ...simply tell them that you are filing for H1B visa and USCIS needs some document/letter from them.
Best of Luck!!