I believe 'when exactly did vampires appear' is one of those questions like 'what caused the first of the kindred to rise', in that there's never going to be a definitive answer beyond 'long enough that nobody really knows'. Although considering how much vitae Kindred need to survive we can guess that Kindred likely weren't the original vampires, and Strix-possessed corpses sound about right.

On that note I don't believe we even have much knowledge as to what the relationship between Strix and Kindred is, although 2e seems to lean towards 'Strix=Beast'. That's not even getting into...