Really? I guess it's been a while since I watched EP4, but where is it made explicit that Vader intentionally let the Millenium Falcon escape?
I mean, it's a bit stupid that all they'd have to do is shoot down 6 TIE Fighters to get away, but letting them get away seems incredibly dumb and risky. Vader completely lucked out that the Rebel base was within firing distance of the Death Star. They could have been on the other side of the galaxy, and planned an organized attack with weeks of preparation time.
THey put a tracking device on the Falcon. THere is a scene right after their escape with Vader and Tarkin. Tarkin says "I'm taking a big risk Vader. This had better work" or something like that.