Need advice on combining tables or leaving as separate tables
My advice is allowing PTO to be a sub-type of event.
If Event is a super-type it will have an additional field: type.
If type=PTO, then there should be an item in the PTO table, with the event_id to link them.
You can read about super-type/sub-type relationships on your favorite database information website, but a sub-type minimizes data duplication, and keeps similar data in one place. So, all events have a start and end date, but not all have a reason like vacations do.
Once the tables have been changed, you could add the start and end date from each PTO row to Event, and add the reason and other PTO data to the new, smaller PTO table. Then you could drop the PTO table once all the data has been moved.