Are you using a "Patch Analysis Job" or the VPC NSH Script Job patch analysis jobs ?
The VPC patch analysis should work against redhat 5 just fine. the VPC is a separate download from the site.
I'm using the same nsh script I use to run patch anaylsis against my RH4 servers (it came from the vendor patch download).
which VPC version? the rh4 scripts use up2date, while the rh5 scripts use yum.
i believe in the 7.4.3 VPC you want to use the "Linux Patch Scripts" as opposed to the "RedHat Patch Scripts"
you also need to set the scripts w/ the rh5 repos in the repo config file for the vpc.