usually I use my DV when I need something portable, so without water filtration, but I did try it once at home just to see if it would work.
what worked for me: a piece of silicon tubing(the thickwalled kind you get with an underdog, I had it laying around), one end tapered to fit the 14mm GonG connection of the downstem. slide the mouthpiece into the silicon tube, and th seal is good enough to vape trough the bong.