Likely 1) lack of food/water, 2) deadlifting itself
Just discussed about it a few weeks ago in class regarding fainting mechanics. Not really vasovagal response, but due to the Valsalva maneuver itself, causing a reduction in venous return to the heart.
Vasovagal response causing blackouts in exercise would usually occur after completion of the particular activity, not during (that would sound more like a cardiac pathological cause and needs investigation).
Sure you can optimize your hydration/food if that's the real issue, but I'm not sure how to prevent Valsalva maneuver-related fainting, considering that you need it to optimize core stability during deadlifts.