Question: what season of vampire diaries should you start watching the originals?
Answer: the Vampire diaries should be watched first as the Originals are a big part of season 2–4. From season 4 you can watch both shows concurrently (switching shows when each season is done) or just finish Vampire Diaries and then move on to The Originals. Tvd s1 to s4 and then you can begin watching the originals.