If brushing your teeth with traces of acid in your mouth really is dangerous, shouldn't we hear about it without mentioning of vomit?
Yes, and we do (from Colgate):
You should know, however, that brushing your teeth after eating can sometimes affect your tooth enamel. According to the Mayo Clinic, if you've consumed anything acidic, you should avoid brushing your teeth for at least 30 minutes. Foods containing citric acid, like oranges, grapefruits and lemons, weaken tooth enamel. Brushing too soon after eating them can damage the enamel in its weakened state.
If you don't trust corporate sources, note the Mayo Clinic link that says the same thing. Avoid brushing teeth immediately after exposing them to acid. That source doesn't mention vomiting, but here's a link from a bulimia support group that does mention vomiting. It agrees with your original source. Do not brush your teeth immediately after vomiting. Rinse with water (can add baking soda) instead.
A more formal research overview says
It is concluded that keeping tooth unbrushed for at least 30 min after an erosive attack is necessary for protecting dentin (Attin et al., 2004).
Which it cites as from "Attin T, Siegel S, Buchalla W, Lennon AM, Hannig C, Becker K. Brushing abrasion of softened and remineralized dentin: an in situ study. Caries Res. 2004;38(1):62–66. doi: 10.1159/000073922. [PubMed] [Cross Ref]".
You may only be seeing this in pregnancy sources because of morning sickness. For most people, vomiting is rare enough that it's not going to be a big deal. Bulimics and pregnant women simply vomit more often and therefore get more advice. For most people, fruit (citric) juice is going to be more dangerous.
Note that they aren't telling people not to brush their teeth. Just to wait a while. Or to brush before the acid exposure, which is of course easier with citric acid than vomit.