There is a more simple approach. With "vorallem", look up "vor" and "allem" in the dictionary, and if the added frequency of both is high enough, then put the suggestion at the top of the list. This approach could be useful for different languages.
I'm not sure if the frequency in the Morfologik speller dictionary can be read from LanguageTool. In Catalan the frequency is in the tagger dictionary (the only dictionary we have), but in German it is in the speller dictionary, isn't it? Or just take a list with the 100 most common words in German.
Of course, with n-grams you could get better results and detect other kinds of typos.