There's nothing in the original films to indicate when they last saw each other. When the Millennium Falcon is brought aboard the Death Star, Vader says "I sense something. A presence I have not felt since...." he then walks away before saying anything else. Later in the film, when Obi-Wan and Vader are face to face, Vader says "When I left you, I was but the learner. Now *I* am the master.". He doesn't say "The last time we saw each other is when I left you. When I was but the learner..." So there was definitely room for them to meet again.