|Doezis was meant first for Spanish, then for English.|
The main rule for vowels is about joining; for example, if it's better a curve A, then use it. However, for convention, some vowels are used for representing an ending (suffix).
It's easy for writing in Spanish, anyway I don't use it, because I don't like it, mainly, because in comparison with Pitman's system, that doesn't require vowels (and this is a saving-time way in shorthand, especially for a full-of-vowels Spanish); Doezis has twelve strokes for vowels, and they are the longest ones in comparison with the consonant strokes. Don't you think?