Agni is both the sacred hearth plus the deity of that hearth who transmits oblations to one other deities. Consequently, this mantra is recited while invoking the deity Agni in the form of fireplace as witness to your vow. Agne vratapate vrataṃ cariṣyāmi (Vājasaneyī-Saṃhitā with the Śukla [White] Yajurveda one.5): https://defaultdirectory.com/listings13296517/the-smart-trick-of-beej-mantra-that-no-one-is-discussing