This is a good question, but part of the answer lies in a fundamental flaw. In the verse beginning viṣṇu-śaktiḥ parā proktā it is stated that the living entity, the jīvātman, is known as kṣetrajña or the knower of the field consisting of body, mind, intelligence, senses, activities, etc. Thus although jīvātman is also superior in nature (tathā parā) like the viṣṇu-śakti there is a third energy, described as avidyā karma-saṁjñānyā: it perpetrates ignorance and bondage through karma. The concept of avidyā or ignorance is important, because it is only due to avidyā that the embodied soul thinks there can be separateness. This third energy is manifest as the vast prakṛti which forms this visible mundane world. Śrī Kṛṣṇa explains in the Bhagavad-gītā verse beginning mamaivāṁśo jīva-loke that all living beings exist as jīvas who are His aṁśa, or eternally constituted part, and are struggling within prakṛti, battling with the mind and senses.
The example has been given of a monkey trap consisting of a jar or kalash which has a narrow opening and a piece of fruit inside. The monkey reaches to grab the fruit, but while holding the fruit cannot remove its paw. All the monkey has to do is let go of the fruit, but it refuses to do so and thus remains trapped.
For the jīva this "fruit" is the idea that we can be the enjoyer and controller. We are part of śakti and we associate with many worldly roles through the first covering of ahaṅkāra or ego, but although śakti is identified with the wielder of energy as one and the same (śakti-śaktimator abheda) the natural occupation of śakti is as the dependent and predominated side.
So the fundamental flaw in your question is the idea that there can be ignorance in the spiritual world. It is a different realm entirely, which is self-effulgent and there is no need of sun, moon or fire (na tad bhasayate suryah - BG) and even time does not exist there (na ca kala-vikramah - SB). It is only due to avidyā or ignorance that we can have the idea that we can be separate from God, and the avidyā which enables this does not exist in the spiritual realm of Vaikuntha. Therefore it is completely preposterous to assume that anyone can fall from Vaikuntha. Maharaja Yudhisthira asks a question about the fall of Jaya and Vijaya to sage Narada for this very reason: how was it possible? The answer was that simply it was the desire of Lord Narayana to engage in the lila or pastime of having a good fight, so Jaya and Vijaya temporarily came as daityas to play the role - really well, I might add - of powerful enemies.
Now the real question you should be asking here is this: how is it possible that the jīvātman could end up in embodied existence in this world of avidyā and illusion? There are no direct statements from sruti on the topic of the beginning of the beginningless jīvātman (na tv evaham jatu nasam - BG) and how some jīvātmas could end up trapped in this world of avidyā and illusion. According to Gaudiya Vaishnava ācāryas (authorities), the jīvātman could only have emanated in a marginal region where there was freedom to go to one side (avidyā) or the other (Vaikuntha) and thus there was exercise of free will.
The idea that the jīvātman in the Vaikuntha realm should be free to have relationships which exclude the Supreme Soul is inapplicable as it depends on the illusory concept of our very existence. It's a bit like the left hand saying to the right hand, "Hey, let's detach from the body and run off and hold onto each other, okay?"
There is no concept of God being an angry and punishing being who wants us to suffer. There are many statements (such as Kuntidevi in Srimad Bhagavatam) to the effect that the various avatars within this world are to benefit the jīvātmas and enable their return to the abode which is effulgent, blissful and eternal. Punishment is found within this world when there is misuse of our independence, but overwhelmingly it has been the express desire of Bhagavan and the saints and sages to lead us toward that self-effulgent existence: tamasi ma, jyotir gamaya.
