The following image explains some of the uses of vowels and wind direction when attempting to build range. Simply put, the vowel sound associated with a range will help the tongue direct the type of wind stream needed for the range to the embouchure. Students may find they need a different wind stream or vowel sound, depending on their physical set up when playing trombone.
The wind direction can also vary. Some students may use an upward wind column when buzzing through the embouchure. I find this style of playing difficult for many reason, but it is a way of accomplishing the same process.