Vernünftig can be both for people and for things; sinnvoll is more for events, abstract notions; vernünftig is a character trait which sinnvoll can not be

Ivo has answered the question very good. A rule of thumb: vernünftig for a person, otherwise they are pretty much used as synonyms. There is a German-German dictionary with good examples and definitions: https://www.duden.de/rechtschreibung/vernuenftig

Wow, that's a good question. I would say, vernünftig has a broader meaning and it refers more to people's mind. While they might be interchangeable in some instances (eine vernünftige Entscheidung / eine sinnvolle Entscheidung), they have a slightly different meaning. "vernünftig" would stress that common sense was applied. "sinnvoll" carries the meaning of a logical decision. Also, "vernünftig" can describe a person ("ein vernünftiger Mensch") while "sinnvoll" can't.

No way synonyms

Nein Vernunft ist eine Art sich zu Benehmen was sinvoll sein kann - Situationsbedingt- sicherlich oft oft ein angepasstes Benehmen ist - muss es aber nicht