The vowel system of Avesta presents many irregularities. In several Avestan words, the vowels are very different from those of cognate words in related languages, even when the consonant structure is identical. In this paper, we suggest that several difficulties with the Avestan vowel may stem from irregularities in the graphic system. In particular, we suggest that when vowels where incorporated into the Avestan script, they were modeled not only on existing alphabets like Greek, but also on syllabic writing systems. Some of the irregularities in Avestan vowel orthography may be vestiges of a syllabic writing system, which may have been the common source of the Avestan vowels, the Old Persian script, and the Indian syllabic scripts.