Unlike vendor lock-in, vendor control implies that by outsourcing a big part of your operations management to a third-party, you also relinquish control over how these operations are handled. This includes the service limitations, the scaling mechanism, and the potential optimization of hosting your application.
In addition, the vendor has the ultimate control over the environment and tooling, deciding when to roll out features and fix issues (although in the case of Azure Functions, you can help fix issues by contributing to the open source project).
