There are two dialogue presets in VisLM, I'm not at work to check exactly what they are named but I've been using the first one. Just leave it on your whole mix as usual and it will measure what it thinks is dialogue, you might see it effected by other things every now and then but that just has to factor into the reading. You're still using it to measure the whole mix, not the Dx stem.

Other than that, just have a well calibrated room and build your mix around your dialogue sitting at this level. I checked a few of my old mixes I did to -24 and they all pretty much followed the new spec anyways. I've actually taken a huge liking to this spec with the show I'm on now, it had an episode that was essentially 50% fight scenes and I know I would have had to make sacrifices to the fights to have it pass at -24 and it was fine under the new spec. (I think it came in around -22 or -21 on my old meter)