Your vendor built the infrastructure and your team now operates it. Infrastructure is a running system, not a deliverable, so a contract that ends at handover moves the expensive half of the work without the knowledge. The three arrangements that do work, the six handover artifacts that transfer real capability, the rebuild test as an acceptance gate, and what an in-house engineer can push for from below.
Do You Need a DevOps Engineer or a Sysadmin? Write the Work Down
The DevOps engineer vs sysadmin question is unanswerable as asked. A hiring guide for 5-to-50-person companies: the four clusters of ops work that get confused, a one-hour diagnostic that picks the role for you, why on-call maths kills single-hire plans, BLS wage data for the occupations these titles actually map to, and when a managed provider beats a hire.