It is strange to even talk about custom reports based on vulnerability scans from Tenable products.
Just look at the variety of report templates available for SecurityCenter. For every taste and need! Why may you ever need to make any custom reports?
The thing is, if you want to use SecurityCenter reports you need to have all the scans of all your hosts in SecurityCenter and, as you know, SecurityCenter is licensed by IPs. What if you have tens of thousands of hosts? Price may be extremely high. In the other hand, you can take Nessus or SecurityCenter scan results via API (read how to do it in “Retrieving scan results through Nessus API” and “Tenable SecurityCenter and its API“) and process it with your own scripts for free.