I think it's futile to try to find a one-to-one translation from German to English. Just because German has Vormittag doesn't mean English should have an exact counterpart, which I don't think it does.
None of the English expressions such as mid-morning, forenoon, late morning is an exact counterpart of Vormittag.
In fact, Wiktionary defines it as all of the three:
Vormittag
- mid-morning, late morning, forenoon (time of day roughly between 9 a.m. and noon)
I think late morning is the closest but it can be entirely subjective depending on who you're asking when late morning starts.