I've been reading A Bolt of Lightning from the Blue recently. I find the part where he writes about the date of the text a bit unclear.
In the preface, Boord argues that something similar to the text we have today dates from Yeshe Tsogyal's time, when she dictated it to Atsara Sale/Ngam 'bre gsal-le/rGyal-ba byang-chub, who passed it on to Padma Las 'brel rtsal, known also as the revealer of the Khandro Nyingthig (with a gap of around 300 years in between). His argument is based on the archaic language of the text, the structure of the text which features comments from three acaryas (Padmasambhava, Vimalamitra, and Silamanju) and a document written by Padma Las 'brel rtsal titled the mTsho rgyal snyan brgyud phur pa'i brgyud debs about the lineage which immediately follows the 'bum nag in the Nyingma kama. The text itself has no colophon. I think the main area of contention arises from the life of Padma Las 'brel rtsal, which Boord notes as "Dates uncertain".
I don't know if the point about the language is true, but the other claims are pretty much the same as what goes for dating Dzogchen texts or even terma texts in general, although it seems that the 'bum nag wasn't presented as a terma by Padma Las 'brel rtsal. I.e., from a text-critical perspective it seems to me that it is difficult to date the text older than Padma Las 'brel rtsal's time, even though it undoubtedly incorporates older material. However, it also clearly includes native Tibetan influences as well as candali/tummo which is supposed to be "sarma".
So assuming that the 'bum nag dates from Padma Las 'brel rtsal's time, the oldest accounts of Vajrakilaya practice in Tibet would be Sakya Pandita's translation of the fragment of the root tantra which was owned by Padmasambhava, and hagiographies and lineage histories, unless there are Dunhuang documents about it?