I have not come across a vendor management responsible person with a CISSP certificate, but I'm positive that I have only met a small portion of us out there.
However, I am in a similar position to yours. I am our IT back-up person, I handle vendor management, BCP/DR, all policies and procedures, and all risk assessments.
There have been times I have reached out to a third-party IT specific group for support when I have had questions. Not seeing where the CISSP is going to come into play until there are multiple people handling nothing but vendor management for the FI.