Final answer:
Highlight the Single Point of Contact feature of ProSupport Plus, which offers a Technical Account Manager to manage all support needs including multi-vendor software issues, providing seamless assistance.
Step-by-step explanation:
When positioning ProSupport Plus to a customer who is concerned about multi-vendor software support, the feature to highlight is a Single Point of Contact for support. ProSupport Plus provides customers with a dedicated Technical Account Manager (TAM) who becomes the single point of contact for all support needs, including issues related to software from multiple vendors. This means that the customer can have peace of mind knowing they have a go-to expert who understands their unique environment and can efficiently coordinate all support activities for hardware and software issues, even if they involve products from different vendors.