You do NOT need to purchase a Volunteers and Events license for each GiftWorks license. You can purchase a Volunteers and Events license for the number of users that need to use these addins and install the addins on those computers.
Keep in mind that if you use GiftWorks from a computer that does not have Volunteers or Events installed, you will NOT have access to any of the Volunteers and Events features, including:
1) Volunteers and/or Events icons
2) Volunteers and/or Events in the Settings area
3) Volunteers and/or EventsSmartList critera for new and existing SmartLists
4) Volunteers and/or Eventsreports
5) Volunteers and/or Events details when viewing a donor
6) Volunteers and/or Events Home page widgets
You WILL still be able to see Notes created by Volunteers and/or Events actions.