It's based on what you are most interested in. There is some overlap between the two subspecialties. Vascular neurology will make you a cerebrovascular disease specialist in the inpatient and outpatient setting. You will inevitably get training in pertinent imaging modalities. Also, at some places, the fellowship in vascular neurology is designed as a bridge into neurointervention.
NCC focuses solely on the inpatient treatment of acute neurological disorders, not only severe strokes, but also things like MG, GBS, TBI, TSI, coma, etc etc. You have much more internal medicine exposure, and develop more procedural skills. Anecdotally, some radiology and neurosurgery-run NIR departments prefer NCC applicants simply because they feel that it is more rigorous lifestyle/stress wise.
I doubt it makes too much of a difference, so if you're interested in one over the other, you could pursue that. Vascular neurology is 1 year. NCC is 2 years, though technically speaking, the ACGME requirements state that you only need training of 1 year in NCC prior to intervention - I think you would be hard-pressed to find fellowships which allowed you leave your NCC duties early though.