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As I understand it, to them the absolute is a person - Vishnu/Narayana, Rama and Krishna being the most prominent.

What is their stand on the Nirguna Brahman of Advaitists?

I am not interested in their redefinition (if any) of "Nirguna" - just their direct opinion of what Advaitists mean by it.

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 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 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)

What is their stand on the Nirguna Brahman of Advaitists?

Ramanujacharya states this in his commentary on the Brahma Sutras, in the sections titled "The Great Purvapaksha" and "The Great Siddhanta:"

Brahman, which is pure intelligence and opposed to all difference,
constitutes the only reality; and everything else, i.e. the plurality
of manifold knowing subjects, objects of knowledge, and acts of
knowledge depending on those two, is only imagined on (or 'in') that
Brahman, and is essentially false..............

This entire theory rests on a fictitious foundation of altogether
hollow and vicious arguments, incapable of being stated in definite
logical alternatives, and devised by men who are destitute of those
particular qualities which cause individuals to be chosen by the
Supreme Person revealed in the Upanishads; whose intellects are
darkened by the impression of beginningless evil; and who thus have no
insight into the nature of words and sentences, into the real purport
conveyed by them, and into the procedure of sound argumentation, with
all its methods depending on perception and the other instruments of
right knowledge. The theory therefore must needs be rejected by all
those who, through texts, perception and the other means of
knowledge--assisted by sound reasoning--have an insight into the true
nature of things.

So they believe a Nirguna Brahman, as defined by Advaitins, doesn't even exist.