I need some good examples of letters written to inform a vendor that
they have breached their contract on a sales or marketting agreement
in the state of California

Here's an item located at http://www.thecontractstore.com/goods.htm:

Cancellation for Breach Letter ($29.95)
Letter canceling contract for purchase of goods or services for
breach.  Contains UCC statement (where for goods), damages breakdown,
and demand for payment of damages.
     
Attorney Customization ($79.95) 

Have a top attorney customize the Cancellation for Breach Letter
specifically for you or your business.  Comes with form and 1/2 hour
of attorney time.  (California, Illinois, and Florida only; for other
states, contact us.)
 

There's also a simple free form at
http://www.4hb.com/letters/ltrnotcanco.html.  This form is for the
sale of goods, though, not services--you'd have to modify it.