I think it was a mistake not to feature the Mandarin in the second film and yes I know he's hinted at but that's not enough. I would have made Valko a stooge of Ten Rings and made it clear that one of Ten Rings goals is the manipulation and trigger of the Armor Wars.
I think they should have at least stuck a Ten Rings logo on the envelope Vanko received. Something small that you'd have to pause the film to get a good look at.