Nandi is not usually the vehicle of Shiva. One vehicle of Shiva is a goat, who is an incarnation of Vishnu. No clue why he is not mentioned more. He also has a bull, as mentioned later in the quoted text. Vishnu is also the vehicle of Parvati in an undisclosed form (I wonder why)
Sūta said:
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Nārada said:
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Mahādeva said:
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He is Rudra. He is Nārāyaṇa. He is the vechicle of Śaṅkara in the form of a goat. He is the protector of Śiva in the form of a goat.
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He has Śivā (i.e. Pārvatī) mounted upon him.
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Sūta said:
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Brahmā said:
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There Nārada saw the god honoured by deities, seated on the peak of Kailāsa, the god of gods, the preceptor of the world, having five faces, ten arms, and having the trident in his hand, having a skull, having a staff with a skull at the top,1 holding a sharp spear and a sword, holding the trident, fearful, granting boons, and having the bull (i.e. Nandin (probably not Nandin usually)) as his vehicle, with his body (smeared) with ash, having charm due to serpents, and having the (crescent-)moon on his head, resembling a dark cloud, and having the lustre of a crore of suns, and sporting there. (Prostrating himself) like a staff he (i.e. Nārada) saluted the lord of gods.
I love how the text goes from mentioning Rudra as the supreme being to him having a goat as a protector in barely any time.