I've been in similar situations. It boils down to "You hired contractors to do this work because you don't have the skills internally." I cancelled a customer's quarterly release because they didn't hand over their requirements doc by the due date (after 2 months of prodding). The customer got really nasty with me. Got a call from my own VP after a few hours. He started to chew me out for not "working with them". I showed him the dozens of emails and several meetings I had with them being clear about their responsibilities and timelines. He just said "Oh… I'll talk to them." The call with the customer's PM the next day was hilarious. After he had chewed me out in front of 100+ of his people, he had to fall on his sword and take responsibility for messing up their release since it was his responsibility to manage their requirements and get them to me.

I know nobody cares, but it feels good to get that story out!