Bhīṣma's status as one of the Vasus is generally accepted by the omniscient narrators in the Mahābhārata. The narrator, the listener and the reader on the Vaiśaṃpāyana level have no doubts about his status, but the characters of the main story cannot be as sure as the omniscient narrator.
The most interesting part of Bhīṣma's status is his self-identification, or rather the lack of it, as one of the Vasus. In the Ambopākhyāna (5.170-193), narrated by Bhīṣma himself, the Vasus come to Bhīṣma three times: to catch him when he is shot down from his chariot by Rāma Jāmadagnya; to inform him about the prasvāpa weapon in a dream; and to prevent him from using it on the battlefield.
In neither of these occurrences, Bhīṣma as the narrator or as the protagonist of the narrated events identifies the eight mysterious brāhmaṇas as the Vasus and himself as one of them. As the narrator, he only quotes the forefathers of Rāma Jāmadagnya who identify Bhīṣma as one of the Vasus here (5.186.17-20).
Similarly, when the Vasus appear above Bhīṣma's deathbed in the Śāntiparvan, they are not identified by Bhīṣma himself, but by Kṛṣṇa (12.51.15-16). In this paper, I show how the Mahābhārata establishes a fact (Bhīṣma is one of the Vasus) and then works with it in a surprisingly realistic and complex way throughout the text, especially taking into account the limitations, motives, and sources of various characters/personal narrators.