The latter work was once in details studied by Hazra (who listed 4 mss. of the work, cf. ref. below  p. 70 n. 14, 72 n. 18, 138 n. 40), at the time it was not yet edited.
Hazra tells us that the Râma legend "constitutes almost the half of the present extensive text" of this purâ.na. And he presents it (pp. 138-150) on the basis of one of the two IO ms. (no. 1001) mainly because the Asiatic Society ms. was incomplete ("has lost more than half [of Râma legend chapters] from the end", p. 138 n. 40).
I fear (it should be checked) that the editio princeps of the VahniP which was published by Anasuya Bhowmick in 2012 (see ref. below), was based on the Kolkata AS manuscript only. I understand that they now have found a new (more complete) ms. in the Samskrita Sahitya Parishad. The episode referred to of the killing of the Bhârgava Kavi Uzanas (Zukra)'s wife during the deva-asura war and the subsequent curse on Vi.s.nu (the killer) to be reborn as Raama is given in Hazra's account of the VahniP (cf. p. 144).
The 6th century dating would be based on the (Hazra's) 6th century dating... of the Vahnipuraa.na, not on the manuscript itself.
• Hazra 1953-1954 « Studies in the genuine Āgneya-purāṇa alias Vahni-purāṇa », Our
Heritage 1/2, pp. 209-245, and 2/1, p. 77-110, re-issued in Dr. R.C. Hazra Commemoration Volume (Selected Articles of the Late Dr.
R.C. Hazra), Part I, Varanasi, All-India Kashiraj Trust (= Purāṇa 27/1), 1985, pp. 67-122, 122-171.
tathā Āgneyapurāṇam / sampādikā, Anasūyā Bhaumika. --
Kalakātā : Di Eśiyāṭika Sosāiṭi, 2012. cxlii,
610 p. ; 24 cm. -- (Bibliotheca Indica series ; no. 336) ISBN 9788192219585
I'd be interested to know more (indologically) about this report:
Does anyone have the inside track?
Professor Dominik Wujastyk*
Singhmar Chair in Classical Indian Society and Polity
University of Alberta, Canada
INDOLOGY mailing list
email@example.com (messages to the list's managing committee)
http://listinfo.indology.info (where you can change your list options or unsubscribe)