How is it that we deal with monopolized vendors with regard to price quality delivery etc?

by always thinking how you can improve the vendors and your situation (for this you need to have the backing of your management)

example is:

1- when identifying SLA's, identify them to improve your process and not to penalize the evndor.

2- also add incentive SLA's which if reached give the vendor a bonus

3- try to understand the inner setup of your vendors company, most  requests he cannot fulfill are related to his internal structure and not to his unwillignness to help. (unless he has a negative mindset which if proven to be the case you should excahnge him at the next chance)