I'm going to test it out. I'll probably be smoking some weed tomorrow, and I'm having a good week right now (I've been successfully recalling very longs dreams, over an hour each time, for the past few days), so I'll see what my dreams are like tomorrow night. Cannabis does inhibit REM cycles, but you actually get a REM rebound the next day. So you might not have vivid dreams when the THC is still in your system, but when it dissipates you'll likely have much more vivid dreams.