In my experience, discovering that I'd vomited has often been the only sign that I'd had a severe nocturnal hypo, so hypos & vomiting are so very clearly linked I've never spent any time wondering why.
If your stomach is empty the vomit can be very clear & hardly noticeable once dried, so sometimes I've only realised I've had a hypo when I've later discovered a strange knotty clump in my hair. On other occasions I've been just conscious enough to know I was retching & drooling while hypo.
Since this is such a common experience during severe hypos it has always worried me that I might choke on vomit, but perhaps the saving grace is that in order to get so severely hypo, & for it to come on slowly while one slips into sleep or unconsciousness, one usually must have eaten far too little of anything for quite a while, so one typically has an empty stomach. As a result the vomit can be mostly just clear & almost odourless slime. Or it is by the time you notice it if you do at all