President Obama made three so-called recess appointments to the National Labor Relations Board (NLRB) while the Senate was not in recess. These appointments were unprecedentedno other president has made recess appointments while the Senate was in session.
The President shall have Power to fill up all Vacancies that may happen during the Recess of the Senate, by granting Commissions which shall expire at the End of their next Session.These appointments were not to fill a vacancy, they were newly created positions IIRC. The language is clear "that may happen" means there is a person in the position and suddenly leaves, dies, or otherwise can not perform their duties in that position, then the President can make an appointment to fill that sudden vacancy. I would argue past recess appointments as well if the position was open when the Senate was in session and the President waited to fill it during a recess.