I wouldn't imagine it to be the Phos Hilaron, which in terms of time-of-day contains no reference to dawn but only to evening twilight (specifically, φῶς ἑσπερινόν, phôs hesperinón - a linguistic allusion to the Greek deity personification of the evening star of Venus, Hesperus - whence the Latin Vesperus...Vespers in English). "Red light of dawn" is a linguistic allusion to the Greek deity personification of morning twilight, Eos (Aurora in Latin) - so a text using that might be a Lauds (or combined Matins-Lauds - as in the Middle Ages they were often combined) text, not a Vespers text.

It might help to have more of the text - or is it simply a phrase you are remembering (and therefore could be remembered incorrectly)? I can't even find anything in Google Books with those opening words as a phrase.

I’m kind of annoyed though: that article is wrong (not uncommon, but this is egregious) as the author uses the Bute breviary (which is sometimes a loose translation and not strictly the Roman breviary) which is the Tridentine breviary as revised to make a point about the restoration of the text, which was not normative in the Divino Afflatu reform, and the Solesmes editions don’t take up the Hymni Antiqui even in an appendix. I suppose that they weren’t supposed to be generally used (having looked at the Vespers hymns, I think that this was mostly a mistake).