* From the Puranic Point Of View.:
During the Tripura Dahan episode, Lord Vishnu assumed the form of a bull to stabilize the chariot made of Sun, Moon, Seshnaag, etc which couldn't handle the the weight of Bhagawan Shiva.
Sri Linga Purana 1.72.:
28-31. Eulogised by the sages and saluted by the bards the shining splendid lord, the bestower of boons occupied the chariot glancing at the charioteer. The groups of Apsarases, skilled in dancing, danced in his presence (to honour him). When he got into the chariot, evolved out of the different material, the horse originating from the Vedas fell headlong over the earth. The lord Dharaṇīdhara (uplifter of the earth i.e., Viṣṇu), assuming the form of a leading bull lifted up the chariot for a while and tried to stabilise it. But at the next moment even that leading bull slipped down to the earth on his knees.
Then after the destruction of Tripuras, in the same chapter only, Lord Vishnu asked the boon to be Bhagawan Shiva's Vahan. Which he granted happily.
- Lord Viṣṇu also bowed down to Maheśvara. Joining his palms together in reverence he said thus to the three-eyed lord accompanied by Umā.
174-175. “O lord, be pleased with me. O lord of Devas, obeisance be to you. I perpetually desire to be your vehicle. I wish for your devotion as well as my efficiency to bear you. O Śankara, the bestower of boons, I wish for omniscience and all-pervasiveness.
On hearing their submission Parameśvara, Mahādeva, Bhava engaged them respectively in charioteership and the position of vehicle.
Sri Padma Puran 6.71.195b-196a,197a.:
He is Nārāyaṇa. He is the vechicle of Śankara in the form of a goat. He is the guard of Śiva (Kailāsa) in the form of a goat (mesha/rama).
He has Śivā (i.e. Pārvatī) mounted upon him.
So, as Lord Rudra's Gana, he dwells on Mountain.
* From The Vedic/Yogic Point Of View.:
Mata Uma = Lord Vishnu.
Rudra Hridaya Upanishad.:
Rudra is the embodiment of all Devas. All devas are merely different manifestations of Sri Rudra Himself. On the right side of Rudra, there is the sun, then the four-headed Brahma, and then three Agnis (fires). On the left side, there exist Sri Umadevi, and also Vishnu and Soma (moon). (Rudra hridhaya upanishad).
Uma Herself is the form of Vishnu. Vishnu Himself is the form of the moon. Therefore, those who worship Lord Vishnu, worship Siva Himself. And those who worship Siva, worship Lord Vishnu. Those who envy and hate Sri Rudra, are actually hating Sri Vishnu. Those who decry Lord Siva, decry Vishnu Himself. (Rudra hridhaya upanishad).
Rudra is the generator of the seed. Vishnu is the embryo of the seed. Siva Himself is Brahma and Brahma Himself is Agni. Rudra is full of Brahma and Vishnu. The whole world is full of Agni and Soma. The masculine gender is Lord Siva. The feminine gender is Sri Bhavani Devi. All the mobile and immobile creation of this universe, is filled up with Uma and Rudra. The Vyakta is Sri Uma, and the Avyakta is Lord Siva. (Rudra hridaya upanishad).
As a Nourisher, Lord Vishnu is generally considered as a Cow, as Mother Cow nourishes us with her milk and milk products.
Atharva Veda X:10:30-32.:
“The Cow is Heaven, the Cow is Earth, the Cow is Vishnu, Lord of Life. The Sādhyas and the Vasus have drunk the out-pourings of the Cow. When these, Sādhyas and Vasus, have drunk the out-pourings of the Cow, They in the Bright One's dwelling-place pay adoration to her milk. For Soma some have milked her: some worship the fatness she hath poured. They who have given a cow to him who hath this knowledge have gone up to the THIRD region of the sky”.
"Hero of Heroes, Agni! Thou art Indra, thou art Viṣṇu of the Mighty Stride, adorable: Thou, Brahmaṇaspati, the Brahman finding wealth: thou, O Sustainer, with thy wisdom tendest us".
Rig Veda 10:5:7.:
Agni, our first-born of Holy Order, the Milch-cow and the Bull in life's beginning”.
Shatapatha Brahmana 1:7:3:8.:
“That (offering) then is certainly made to 'Agni,' for, indeed, Agni is THAT god;--his are these names: Sarva, as the eastern people call him; Bhava, as the Bâhîkas (call him); Pasûnâm pati ('lord of beasts,' Pasupati), Rudra, Agni. The name Agni, doubtless, is the most peaceful, and the other names of his are not peaceful: hence it is offered to (him under the name of) 'Agni,' and to (him as) the Svishtakrit”.
“Among all domestic animals, thou (Rudra) art the bull that is worshipped by all people”.
So, though normally, Vishnu/Uma is Cow and as being Rudra in Ardhanarishwara form he is also Bull.
Our Spine is called Mount Meru or Meru Danda.:
Uttara Gita 2:13.:
“Like the backbone of a Veena, or harp, the long tract of bone with many joints that stretches from the seat right up to the head of a human being is called the Meru-Danda (spinal cord). There is a minute aperture or hole that passes right through this Meru-Danda from the Muladhara to the head; it is through this hole that there passes a Nadi which the yogis called the Brahma-Nadi or Susumna”.
Narayana Suktam verse(s) 12.: Taittiriya Aranyaka 10.13.2.12.:
"Paramatman dwells in the middle of that flame (Sushuma Nādi). (Although He is thus limited) still He (Narayana) is the four-faced creator Brahmā, Siva, Hari (Vishnu), Indra, the material and efficient cause of the Universe and the Supreme Self-luminous Pure Consciousness".
At the base of the root one should remember the divine triangle, the treasure of all effulgence. She brings the crest of the fire (Sushuma Nādi) upward. She is shining like a lightning bolt in the middle of a blue cloud. She is as slender as a nīvāra parrot and yellow like a shining atom. The Supreme Soul is situated in the middle of that crest (Sushuma Nādi), That is Brahma, that is Shiva, that is Indra, that is imperishable Supreme Self, That is very same Viṣṇu, that same is life-force that same is time, that same is fire, that same is moon. One who thus meditates on the Kundalini is thus freed from all the sins.
So, Vishnu/Uma dwells in Mountain/Spine as Kundalini to reach Rudra in Sahasrara.
I hope this clarifies all your queries. Prd..