I know plenty of runners who smoke or vape and have still improved a lot significantly, as well as second hand stories of elites who also smoke a lot and still run really fast (other than just Chris Barnacle). Obviously it probably won't help your running but I doubt it will make a significant difference, or necessarily any difference at all as long as it doesn't prevent you from getting your runs in at a decent pace. That said, all of the weed and running "success stories" that I know first hand are from people who run somewhere between 80-120 mpw and race events 10k or shorter, so it's conceivable that the weed does have a significant negative impact on their lung capacity, but they just put in enough aerobic work so that it doesn't influence their race distances. If you're training for a 5k or 8k off of something like 20-45 mpw, it could be that smoking would give you aerobic damage that your training couldn't offset, but that's pure speculation.
Also, if you have the means a vape wouldn't be a bad investment.