The rule applies to words consisting of one syllable (such as your examples) and longer words where the stress is on the last syllable. Thus we double the final consonants in verbs such as 'forget' and 'transfer', because the second syllable of these words is stressed. With 'vomit', however, the stress is on the first syllable, so we don't double the 't'. Compare 'vomit' (stress Xx) with the word 'permit' (stress xX), for example. We write 'permitting' and 'permitted' to emphasise the stress on the second syllable, but write 'vomiting' and 'vomited' to show that the stress is on the first. I hope that makes sense.