How to ensure vendor relationship if the person to whom we were in touch leaves the company/firm?

Create a dashboard of the measures that you use measure the performance of the vendor and update this and mail it daily to the account manager and their line manager. When the account manager leaves their line manager has all the relevant daily information and can pass this on to the replacement or cover for the account manager while they are being replaced. What is crucial is to maintain the flow of information and to ensure that you have developed more than a single contact at the supplier.