I think I see what the problem is; you have two searches that need to take place to match on ID from Plugin and plugin_ID from the scans. Give this search a try, it may take a bit longer. I'm going to use some of @cmerriman 's written code hope you don't mind.
index=nessus cve=*
| eval ID=coalesce(id,plugin_id)
| eval CVSS_SCORE = cvss_base_score + cvss_temporal_score
| rename cve as CVE
| stats sum(CVSS_SCORE) as CVSS_SCORE values(plugin_name) as Plugin_Name by ID
| appendcols [search index=nessus
| rename host-ip as hostip
| stats list(hostip) as hostips, list(IP) as IP, count(hostip) as HOSTS by plugin_id]
| table ID, Plugin_Name, CVSS_SCORE, HOSTS
| sort - CVSS_SCORE
... View more