Not only can CBD-rich medications help patients relax and unwind without potentially unwanted psychoactivity, there’s also great therapeutic value for PTSD patients. Considering your personal THC tolerance is a good starting point. Most patients won’t experience much, if any, psychoactivity or "high" from a low dose of a medication with the ratio of 4:1 (4 parts CBD to 1 part THC). If you are new to cannabis or have a low(er) tolerance that might be the right approach for you, whereas if you consume THC to some extent or have a higher natural tolerance, you may want to consider a more balanced ratio. As for vivid dreams specifically, that may vary from patient to patient and it’s worth experimenting to your comfort.