Question: how to watch the vampire diaries and the originals and legacies in order?
Answer: You can watch both shows in sequential order- as they aired. 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.