If you go to roles under the administration tab, you can see the roles that you can add/remove/edit. Once you create a role, it is automatically populated in the drop-down within the user creation screen. From the looks of the custom rights under vApp, it doesn't appear that this is possible without taking away the ability to allow the user to create a vApp as well.
vApp
- Access to VM Console
- Change Owner
- Copy a vApp
- Create / Reconfigure a vApp
- Create / Revert / Remove a Snapshot
- Delete a vApp
- Edit vApp Properties
- Edit VM CPU
- Edit VM Hard Disk
- Edit VM Memory
- Edit VM Network
- Edit VM Properties
- Manage VM Password Settings
- Share a vApp
- Start / Stop / Suspend / Reset a vApp
