I read a sleep study a while back that said that THC inhibits REM sleep (you know, when you dream), so if you smoke before bed, like most of us, you don't hit REM sleep until much later in your sleep cycle. So instead of dreaming at the peak of your sleep cycle, you start dreaming after the THC has left your brain, and for some people this is after they have woken up, hence no dreams. Stopping smoking resets your sleep cycle and lets you dream at your normal time. The reason they seem so vivid is either because it's been so long since you reached good REM sleep, or your brain is making up for lost time, but there ya go.