If you set up a Setting Group for the plugin in the AdminCP > Options, you would use something like
Code:
if (($vbulletin->options['customplugin_enable'] == 1) AND is_member_of($vbulletin->userinfo, explode(',', $vbulletin->options['customplugin_votegroups'])))
customplugin_enable would be a yesno field (turn on/off the plugin) and the customplugin_votegroups would be a comma separated list
i'd recommend putting the typical
5, 6, 7 as default for the _votegroups, blank Option Code, and add something like
(comma separated ID list) in the description