How do you Manage a vendor who is not managing a project well?

I’m working with a vendor who we just sent a contract with to take over some of our infrastructure. It is on them to deliver so they are taking ownership of the project and I am serving as the project manager on our side. However, they are a new startup company and don’t actually have a PMO and it’s the pre-sales account manager who’s doing the project management. We were told internally to allow them to manage the project. However, the project plan they delivered is not very detailed, has ambiguous task and does not give us confidence in their delivery. We are accountable on our end to ensure that this project delivers. What is some advice to help this vendor mature in their project management but not take over the project we’ve provided directions on how to perform a work breakdown structure that they find that administratively intense and micromanagement. Any advice is welcome.