Site map resubmission?

If I have a site like example.com/sitemap.xml.gz, and I use vBulletin for an open forum, is it still necessary to install another sitemap generator for example.com/forum/sitemap.xml.gz ? Or the main sitemap should do the job for every secondary/tertiary URL and so forth?

The sitemap for your root is likely not going to contain the urls for your forum.

You should set up 2 webmasters areas, one for root and one for /forums and submit the sitemap_index.xml.gz for the forum, and whatever you are using for the root per their instructions.