A year ago i was told i needed needed more omega 3 + vit d could this have prevented hsp vasculitis. i read somewhere that these prevent autoimmune d.

Doubt it: HSP is a sporadic process that happens for unknown reasons to those susceptible to this disorder. Whether or not vitamins or supplements could prevent or modify the process is speculative. It would be hard to come up with a way to test the theory on such a sporadic event.

Uncertain: omega 3 is considered anti-inflammatory and vitamin D3 an immune regulator. Thus they may be helpful for autoimmune diseases by decreasing the inflammation and bolstering the immune regulation. However there is still a gap between the purported actions vs actual results in modifying disease conditions.