Answer
The service one performs in Vaikuntha is simply service to the Supreme Lord. The residents of the spiritual world are fully engaged in assisting the Lord in His eternal pastimes.
In the spiritual world, no one has any obligatory duty to perform for the sake of maintaining themselves, earning money, fulfilling material obligations, or securing necessities of life. Such concerns do not exist there because everything is spiritual, complete, and fully satisfying.
There is no question of having to work in order to survive or maintain one's existence. The residents of Vaikuntha are eternally liberated souls who are completely free from all material anxieties and necessities.
At the same time, the soul is by nature active. Activity cannot be eliminated, because activity is an inherent characteristic of the living being. Therefore, the question arises: if there are no material duties, what activity does the soul perform?
The answer is devotional service.
The eternal activity of the soul is loving service to the Supreme Lord. In Vaikuntha, this service takes the form of assisting the Lord in His various pastimes according to one's unique spiritual relationship with Him.
Krishna is eternally engaged in His transcendental pastimes, and His devotees joyfully participate in those pastimes by rendering loving service. Their service is not performed out of obligation, fear, necessity, or expectation of reward. Rather, it is a spontaneous expression of pure love.
Thus, Krishna enjoys His eternal pastimes, and His devotees also experience unlimited happiness by cooperating with Him and serving Him in love. The entire spiritual world is centered on this loving exchange between the Lord and His devotees.
Therefore, the service performed in Vaikuntha is not work in the material sense. It is the natural, joyful, and eternal activity of the liberated soul, fully engaged in loving cooperation with the Supreme Lord.
