Dr says I've immune low platelets & vasculitis. 28f lifelong martial artist+runner. If I'm careful, can I still bleed? Are they as serious as I read?

How low? Mild immune thrombocytopenia is basically harmless, a disease of the lab tests. If your platelets are around 50,000 or below, I'd get a couple of opinions. I'd also insist on a more specific diagnosis that "vasculitis". Do you perhaps have lupus? How has the workup proceeded? I'm very glad you're exercising and if impact sports become a problem, consider swimming (my sport).