This was going to go in the last post, but I decided to split it out. Playing with unicode today, expanding on my findings in my post on serela diacritics.

I can get a solid 6x12 chart using unicode accented latin (not combining spaced characters):

a	ã	à	á	ả	ạ
â	ẫ	ầ	ấ	ẩ	ậ
ă	ẵ	ằ	ắ	ẳ	ặ
e	ẽ	è	é	ẻ	ẹ
ê	ễ	ề	ế	ể	ệ
i	ĩ	ì	í	ỉ	ị
o	õ	ò	ó	ỏ	ọ
ô	ỗ	ồ	ố	ổ	ộ
ơ	ỡ	ờ	ớ	ở	ợ
u	ũ	ù	ú	ủ	ụ
ư	ữ	ừ	ứ	ử	ự
y	ỹ	ỳ	ý	ỷ	ỵ

Plus I can get two more characters with some significant subset of those 6 (substitute, non-matching characters in parentheses):
n ñ ǹ ń (ṅ) ṇ
ō ȭ ṑ ṓ (ǭ ȱ)

That's a lot of vowels.