Many questions about cryptography are also about security and so are on-topic here as well as on Crypto.SE. For example, our many questions about password hashing are on-topic on both sites — but they would get different answers: Crypto.SE tends to have answers aimed at crypto designers and implementers, while on this topic Security.SE caters mostly for system administrators and end-users. The general observation about questions on-topic on both sites is about this kind of questions.
A question like “how do I find test vectors for this algorithm” has nothing to do with security. Whether a cryptographic algorithm matches a security policy is about security. Whether a cryptographic implementations is secure against a threat is about security. But getting the mathematical computation right is not about security. “How do I find test vectors for this algorithm” is off-topic here.
https://security.stackexchange.com/questions/133198/nist-test-vectors-for-rsassa-pkcs1-v1-5-sign-with-the-sha-256-hash-function is off-topic here but on-topic on Crypto.SE, so it should be migrated to Crypto.SE.
NIST test vectors for HMAC-SHA-256 is off-topic here but on-topic on Crypto.SE, so it should be migrated to Crypto.SE. However, there is a stupid limitation that questions older than 60 days cannot be migrated. So we're stuck either leaving an off-topic question open, or closing the question in place.
Like several other Stack Exchange sites, starting with Stack Overflow, Sec.SE's scope has tightened a bit over time. In the early days, it was more accepting of off-topic (e.g. non-security-related crypto) or unanswerable questions (e.g. polls), in the name of increasing traffic. Years later, this is a legacy that causes unfortunate effects. (Now where have I seen that before?) There's no perfect solution.