Agni is both equally the sacred fire and also the deity of that fireplace who transmits oblations to the other deities. Hence, this mantra is recited whilst invoking the deity Agni in the shape of hearth as witness on the vow. Agne vratapate vrataṃ cariṣyāmi (Vājasaneyī-Saṃstrikeā from the Śukla [White] https://aglocodirectory.com/listings13301886/the-2-minute-rule-for-mantra