When Entering into a Third Party Vendor Agreement for Security

When entering into a third party vendor agreement for security services, at what point in the process is it BEST to understand and validate the security posture and compliance level of the vendor?

A) At the time the security services are being performed and the vendor needs access to the network
B) Once the agreement has been signed and the security vendor states that they will need access to the network
C) Once the vendor is on premise and before they perform security services
D) Prior to signing the agreement and before any security services are being performed