Hi,
This is a little tricky.
The easiest way to do this would be to have all the events already in the calendar.
When a day is picked all the events for that day would show in a drop down.
The dropdown would be filtered by the day and if the event was available.
When an event is chosen it already has the times so no need to worry about that bit. The form would update the event with a user. When a new form is opened and a day picked the lookup filters on that days events and if there is a user assigned or if all events have a default user any event with that default user is displayed.

Alternative method.
You could have the drop down with the 1-12 and depending on what is selected, Calcualted value controls referencing the dropdown could calculate times and add them to the start and end date columns.
How you would filter the drop down with which vacation is available I am not sure.

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