How can I get vendors to actually deliver what they promise?

You have learned something many people never do: good vendor work is not luck, it is design. When terms are clear and someone is plainly accountable, most vendors rise to meet it. You are not squeezing them. You are giving them a target sharp enough to hit. Keep doing what works, and notice why it works. Clear terms remove the excuses. A named owner removes the confusion. Fair pay removes the resentment. When all three are in place, delivery stops depending on goodwill and starts depending on structure, which is far more reliable. The next step is to protect this as things grow busy. Write down your own standard for a vendor agreement, the few points you never skip, and use it every time. When you are rushed is exactly when the old surprises creep back in. A simple checklist keeps your good judgment from getting lost in the hurry.