RAMANANDI SAMPRADAYA
The Ramanandi Sampradaya (which advocates Vishishtadvaita Vedanta), accepts the Advaitic definition of Nirguna Brahman.
Tulsidas, a prominent member of the Ramanandi Sampradaya, explains, in his "Ramcharitmanas" that Brahman (Rama) is originally Nirguna/Nirakara and only becomes Saguna/Sakara for the sake of Jivas:
There are two aspects of God-the one unqualified (Nirguna) and the other qualified (Saguna). Both
these aspects are unspeakable, unfathomable, without beginning and without parallel. To
my mind, greater than both is the Name, that has established Its rule over both by Its
might. Friends should not take this as a bold assertion on the part of this servant; I record
my mindís own conviction, love and liking. The two aspects of Brahma (God) should be
recognized as akin to fire: the one (viz., the Absolute) represents fire which is latent in
wood; while the other (qualified Divinity) corresponds to that which is externally visible.
Though both are inaccessible by themselves, they are easily attainable through the
Name; therefore I have called the Name greater than Brahma and Sri Rama both.
Brahma (God) is one, all-pervading and imperishable; He is all truth, consciousness and
a compact mass of joy. Even though such immutable Lord is present in every heart, all
beings in this world are nonetheless miserable and unhappy. Through the practice of the
Name preceded by Its true appraisement, however, the same Brahma reveals Itself even
as the value of a jewel is revealed by its correct knowledge. (Bala-Kanda, Ramcharitmanas)
There is no difference between qualified (Saguna) Divinity and the unqualified (Nirguna) Brahma: so
declare the sages and men of wisdom, the Vedas and the Puranas. That which is
attributeless and formless, imperceptible and unborn, becomes qualified under the
influence of the devotee's love. How can the Absolute become qualified? In the same
way as water and the hail-stone are non-different in substance. Infatuation is out of the
question for Him whose very Name is like the sun to the darkness of error. Sri Rama,
who is Truth, Consciousness and Bliss combined, is like the sun; the night of ignorance
cannot subsist in Him even to the smallest degree. He is the Lord whose very being is
light; there is no dawn of understanding in His case (for the dawn presupposes night and
night there is none in the sunlight of Sri Rama). Joy and grief, knowledge and ignorance,
egoism and prideóthese are the characteristics of a Jiva (finite being). Sri Rama is the
all-pervading Brahma; He is supreme bliss personified, the highest Lord and the most
ancient Being. The whole world knows it. (Bala Kanda, Ramcharitmanas)
Like Advaitins, Ramanandis also believe that it is not possible to perceive the Nirguna aspect of Brahman if one is under Maya/Avidya:
Covered by dense lotus leaves the water could not be easily discerned, even as
the attributeless (Nirguna) Brahma is not perceived when veiled by Maya (Ignorance). All the fishes
that had their abode in the fathomless water of the lake were uniformly happy, even as
the virtuous ever pass their days peacefully. (Aranya Kanda, Ramcharitmanas)
GAUDIYA-VAISHNAVA SAMPRADAYA
Jiva Goswami, the renowned Gaudiya Vaishnava acharya, in his "Tattva Sandarba" finds the Advaitic Nirguna Brahman to be fictitious:
In this way, by basing their ideas on Brahman and Avidya alone, the Mayavadis
contradict themselves when they say that the one undivided Brahman (Nirguna Brahman), pure by
virtue of being unadulterated consciousness and thus altogether free from contact
with avidya, is nonetheless polluted by contacting avidya and thus becomes the
jiva Then again, say the Mayavadis, that the same Brahman becomes the personal
Godhead when He serves as the basis of Maya, the illusion concocted from the
Jiva's avidya. And under the influence of Maya, Brahman supposedly once more
becomes the Jiva. Here we have avidya within the pure spiritual being (Brahman),
vidya within the upadhi called God, who is concocted by that avidya, and an
illusory status of that same Godhead, who is the proprietor of vidya. We should
carefully study how these and other similarly manufactured ideas are simply
incoherent.
However, Gaudiya Vaishnavas do subscribe to an impersonal Brahman which is simply seen as the bodily effulgence of Krishna.