SScooter

As I was driving down the highway a few days ago I noticed a bad vibration and I put my car in neutral I did not apply the brakes but I could feel the car slowing down rapidly. I couldn't figure it out so I pulled the vacuum line off of the brake booster and plugged it back in. This solved my problem for about two days now I'm back to the same problem. Anybody have a clue why it would do this?