Project managers (the customer end and the vendor end) have to work hand in hand during the implementation stage of a software project happening at customer site. The key responsibility of both the project managers working on the project is to ensure successful implementation and project closure.
It is the customer project manager who has to take the charge, be in full lead and command to drive the project and keep the responsibility and onus to him and his management. The implementation plan has to be his baby all the time. Unless and until he takes the lead, he will not be able to put the potential fire in his team to accept the product. And then only he will be able to ensure a fully hassle free implementation.