Can vascular surgeons diagnose root cause of vascular problems (such as autoimmunity and/or infection)? are they trained to diagnose/treat root cause?

See below: In terms of autoimmune diseases, they can perform a temporal artery biopsy which can confirm the presence of temporal arteritis, but the rheumatologists usually manage the autoimmune causes.

Yes: Vascular surgery training ensures proper diagnosis of the vascular disorder before treatment. The vascular lab is essential for this.The treatment is often not the same for an autoimmune or infections etiology as compared to atherosclerotic disease. A large number of patients seen in consultation are treated medically. Open surgery and endovascular interventions are performed by vascular surgeons.

Yes: Vascular Surgeons are experts in the diagnosis & management of vascular conditions, including autoimmune vasculitis & infectious vasculitis, which most commonly lead to either aneurysms of arteries or blockages within arteries, but can also affect the veins. Many of these conditions do require vascular surgery procedures. Vascular surgeons are not just "surgeons", but they also treat medically.