In most cases yes, since a P&ID graph that the company has already built and maintains is a valuable source of truth, and rebuilding it separately creates a second version that will drift. Consuming the existing graph keeps one model of the plant that everyone uses.
The practical question is whether the existing graph has the detail and quality the vendor needs, and how updates will flow when it changes. If gaps exist, the better approach is usually to fill them in the existing graph rather than build a separate one, so the company keeps ownership of its own data.