But then that begs another question - at the end of Rama's reign, why would the devas & Brahma have sent Mahakal to Rama to persuade him to end his avatar - maybe taking a bit more time if he so desired? If Vishnu was always in Vaikuntha, there is no way the devas would have 'missed' him.
In fact, the first 5 avatars of Vishnu - Matsya thru Narasimha - Vishnu just took those forms for a short while before reverting to his normal self (I'm going by the dasha-avatar theory, not the 22 avatar of Vaisapayana) while from Vamana onwards, he actually took human births and lasted for years. I've never read of Vishnu being there in Vaikuntha doing squat - he was always where the avatar was, whether it was visiting Bali, wiping out kshatriyas as Parashurama, or as Rama and Krishna.
In the case of Vamana, Vamana, as a child, visited Bali and reclaimed the worlds from him. Parashurama was long lived, but his avatar may have ended after either his encounter w/ Rishi Kashyap asking him to end his wars, or his encounter w/ Rama after the latter's marriage.
In case of Rama, Brahma sent Mahakal to him to invite him to return to Vaikuntha. In case of Krishna, the 2 curses the Yadavas received - from the rishis and from Gandhari - saw to their destruction as well. In both cases of Rama & Krishna, the devas saw to it that they got a fitting journey back to Vaikuntha, all in all suggesting that Vaikuntha was vacant during their lifetimes.