If one wanted more Ki, it seems pretty tempting to class in monk for Ki/day=WIS, and some in Ninja for Ki/day=CHA. And then add in monk vows, and it seems you can expand that number.
To somewhat balance out the burden, it seems that the Ki from vows scales with level. In RAW I've seen this specifically tied with monk level, but my gut tells me it should be character level. What's the latest understanding of this rule?
RAW it would not, but if I were DMing I'd let it level with character level since it's a penalty that never goes away.
It is definitely Monk level. Individual GMs might houserule it, but there's no question about the actual rules.