Figured out the problem without the VNA. Turned out I have a 50kW FM station a bit less than 5 miles away overloading my tuner. An FM band trap solved the problem. In a forum for the tuner someone suggested doing a scan with it set to cable rather than over the air channels which included the FM band and they were able to see the FM strong station - a case of hardware not really made for the job but doing it well enough to see the issue.