How come many patients with clinical eds diagnoses do not test positive yet have vascular issues?

They test different vessels. They're testing different blood vessels than the ones people with ed have involved. There are different types of blood vessels. Some are different by size, some are different by composition, some are different by function. The ones involved in ed cannot be measured.
No simple test. Eds in one form (vascular eds) can cause weakness of the blood vessels. If the vessel ruptures seriousl consequences can occur. Eds causes a weakness or loss of elasticity which can lead to stretching, ballooning out and eventual rupture. Probably easiest way to diagnose is with ultrasound or ct with contrast.