I know how "Moksha" is explained in both "Advaitha" and "Dvaitha" philosophies.
What about "Vishishtadvaitha"?
I've summarized the relevant parts from A Catechism of the Visiṣṭhādvalta Phllosophy of Śrī Rāmānuja āchārya by N. Bhashyacharya.
What is Moksha?
Entire separation of Jīva from all connection with matter, and complete destruction of Karma whether good or bad. The word Mokṣa literally means release from bondage (bandha). All Jīvas can obtain Mokṣa if qualified to do so.
Means to attain Moksha
There are four means: Karma-yoga, Jñāna-yoga, Bhakti-yoga and Prapatti(Saranagati/devotional surrender).
Only persons of great mental development and who possess Jñāna (spiritual knowledge) are fit to practice the first three, viz., Karma-yoga, Jñāna-yoga and Bhakti-yoga. Those who have attained neither mental development nor spiritual knowledge, and who are incapable of practicing the other means of mokṣa may have recourse to Prapatti. This may be practiced by any one, without distinction of caste or creed, even if he be a chandala or a mleccha.
Kinds of Moksha
Moksha is of 2 kinds, Kaivalya and Brahmānanda. Kaivalya is an isolated form of enjoyment whereas Brahmānanda is enjoyment in a relationship with Parabrahman.
Where is Moksha?
It is enjoyed in a place called parama-pada (supreme state or Vaikuṇṭha loka), which is not material, being comprised of śuddha-sattva. It is above all material worlds. Both kinds of mokṣa ’Kaivalya’ and ’Brahmānanda’ are enjoyed in Vaikuṇṭha loka, but in different parts of it.
Is Moksha permanent?
Mokṣa is everlasting. Muktas (Jivātmas that have attained Mokṣa) are never again subject to the qualities of matter or to Karma. But if they choose, for the sake of doing good to the world, they may incarnate on earth. After such an incarnation they return to Mokṣa, and even during these incarnations they are not subject to matter or Karma.
In Vishistadvaita a Jiva has two kinds of attributes, Dharmi-jnana and Dharmabhuta-jnana. Dharmi-jnana may be described as intrinsic consciousness of the subject revealing itself and Dharmabhuta-Jnana as attributive consciousness which reveals external objects to the subject. In bondage the attributive consciousness of the Jiva is contracted or very limited in scope of its functioning.
The liberated Jivas are those who, evolving through all forms of bodies, at last come to the stage of human birth, and eventually attain moksha through spiritual disciplines and surrender to the Divine. By the grace of Narayana they are liberated from Samsara and raised to the Divine status of Vaikuntha, where they get the new spiritual body of Suddha-satva. Being free from the load of Karma, their Dharmabhuta-Jnana now attains to its maximum expansion and becomes one with that of Ishvara. But their atomic nature (anutva) remains, distinguishing them from the Infinity (Anantam) that God is. In the Divine Vaikuntha they become His servitors or instruments of service, or remains absorbed in the bliss of Narayana. They become one with God in knowledge and bliss, but not in power; for Narayana alone is the Great Cause of the universe (liberated Jivas do not have the power to create, preserve or destroy the universe).
REF: Bhakti Schools of Vedanta by Swami Tapasyananda