Are strokes or tias caused by vasospasms?

Rare but possible. Vasospasm in the brain occurs when blood and its breakdown products are present in the spinal fluid around blood vessels. The most common reason is rupture of brain aneurysm, but there are other conditions. Vasospasm causes the larger vessels (arteries, arterioles) to clamp down or contrict, thus reducing blood flow. This can cause strokes or tias.
Possible. While the answer is technically yes, most strokes and tias are not caused by vasospasm but rather by plaque rupture, platelet aggregation, and thrombosis.