Reddit reviews Vital Circuits

Here is a whole book on the circulatory system for you.

It explicitly mentions “Vascularity is enhanced by extremely low body fat (usually below 10%) and low retained water, as well as the muscle engorgement ("pump") and venous distension accentuated by the vigorous flexing and potentially hazardous Valsalva effect which characterize competitive posing. Genetics and androgenic hormones will affect vascularity, as will ambient temperature”

Vascularity is a real thing and it is most effected by lower body fat.