If they haven't recorded it in some way then it doesn't matter. And if they have, then it's what a human historian could do too.

Can you recall well what you did ten years ago to the day? How much is a vampire is going to accurately recall from a hundred years ago?

At best, the vampire has bragging rights like "I hung out with Gandhi."

We'd need some evidence the vampire has super-recall AND is somehow trustworthy enough to not change the story depending on motivations of the day.