OmniPlan's leveling assumes that a task with multiple resources assigned to it can be worked on by any one resource whenever possible. If you know the vacation days of one resource, the only workaround I can think of is to manually set a locked start/end date for the meeting task that you know will work for the Resource who has vacation days.
Then, you can let leveling sort out the scheduling of the second Resource that may not have any vacations scheduled. This will get complicated if your project has multiple resources but unfortunately that's all I can suggest until we add the ability to specify that a task must be worked on by all resources concurrently.