Is Advaita moksha different from attaining Vaikuntha? If yes, why and is this why Advaita is often frowned upon by most Vaishnavas?
Yes, Advaita moksha is different from both dvaita and Vishishtadvaita Vedanta.
This school believes both in moksha while still living (Jivanmukti) and after death (krama-mukti). It is not possible to go any further since Advaita moksha is non-dual in nature. Nevertheless I am posting the following passage:
Bhishma continued [Suka said],’..By knowledge, one attains to that whither there is no occasion for grief; whither one becomes freed from birth and death; whither one is not subject to decrepitude; whither one transcends the state of conscious existence; whither is Brahma which is Supreme, Unmanifest, immutable, ever-existent, imperceptible, above the reach of pain, immortal, and transcending destruction; whither all become freed from the influence of all pairs of opposites (like pleasure and pain, etc), as also wish or purpose. Reaching that stage, they cast equal eyes on everything, becoming universal friends and devoted to the good of all the creatures.’
Mahabharata Santi Parva Section CCXLI
According to this school moksha is only possible after death. In ascending order, this school believes in four levels of moksha: (1) salokya, (2) samipya, (3) sarupya and (4) sayujya. ….. In salokya mukti, the departed soul goes to Ishta-loka (the abode of Vishnu) and stays there blissfully enjoying His presence. In samipya-mukti the departed soul enjoys the bliss of extreme proximity to Vishnu. In sarupya-mukti the departed soul acquires the form of Vishnu and enjoys intense bliss. In sayujya-mukti the departed soul becomes blissfully absorbed in Vishnu.
The school of Qualified Non-dualism or Vishishtadvaita-vada, is a school of theism. Its main exponent was Ramanuja. According to this school, a person can have moksha only after the person’s death. Moksha means living blissfully in Vaikuntha (the abode of Vishnu) in spiritual bodies. They acquire many divine powers such as omniscience, but unlike God they cannot create, sustain, or dissolve the world. In spite of their exalted state they remain subservient to God.
Why do Vaishnavas frown upon Advaita moksha? The answer is that Vaishnavas think that in Soham (I am He ) He stands for Vishnu. This is because Sri Vaishnavas do not accept the concept of Nirguna Brahman as understood by Advaita.Sankara of course considers Nirguna Brahman to be real and Saguna Brahman to be unreal. Thus Advaitins reject that idea that Soham means 'I amd Vishnu'.They instead hold Soham to mean 'I am Nirguna Brahman'.