Month ago I had a concussion. How long is the brain vulnerable for a second? I breaked hard while driving 10 mph. Could that cause concusion again? Thanks

The brain is like. A fancy high tech jello mold. It moves inside the skull so yes, it is possible to have a concussion without actually striking your head. If you were dazed after the incident it may have been a problem.
Not really. The brain is always vulnerable. I do not think that such a minor acceleration/deceleration exacerbate another concussion. Usually symptoms from a concussion resolved within 6 to 8 weeks.