Complex sentence: Vowels contain a more modest number of phonemes, with the average being 5-6 and comprising of 51% of the languages in the survey, and about a third of the languages have larger than average vowel inventories.
Split into simpler sentences: Vowels contain a more modest number of phonemes, with the average being 5-6, which 51% of the languages in the survey have. About a third of the languages have larger than average vowel inventories.