Vietnamese uses a high number of diacritics. Are there any languages that are even higher, in terms of either the maximum or the mean of the number of diacritics added to letters?
If we are speaking of current official spelling systems, I believe that Vietnamese wins the prize. Potential competitors would most likely be a language with a rich vowel system than included independent tone, phonation and nasalization contrast, which points to Ju|'hoansi and !Xóõ, which would have a higher maximum and probably a higher text frequency – if these languages were transcribed in IPA. However, Ju|'hoansi has been undergoing a progressive reduction in diacritics to the point that the 1994 orthography is diacritic-free (tone isn't marked). !Xóõ orthography likewise trades in diacritics for VC digraphs.