Hey Colter, thanks for reaching out! Yes, they do, but only on the wide-mouth bottles. Hydrapak makes the majority of the soft flasks you see on the market right now so it'll depend on the exact model of bottle that came with the running vest you have. I have used this filter on an Ultimate Direction Body Bottle Plus
and the diameter/thread pitch works perfectly! However, it will not fit on the smaller CAMP
flasks that I've seen come included with those packs.
I have found that the only bummer with using it on your existing .5L bottles (as opposed to buying the filter flask separately, which has .6L of usable volume) is that the filter takes up a pretty good chunk of volume in the flask, so you are really only carrying .4L, maybe .45L, on the high end within the bottle. Also, if you are drinking from some really nasty water, it might be good to match the filter with a specific flask, that way you don't put the normal cap back on your flask and drink contaminated water that was left over from your last use with the filter.