I think you should see it like this, all bleach captains are expected to have a bankai, but Kenpachi became captain without bankai but was able to become captain because he had other qualities.
It has been mentioned that all vice-admirals have haki, but Smoker with a logia body and a seastone Jutte has enough quality to make it as a vice-admiral even without haki.
It has been mentioned that all vice-admirals have haki, but Smoker with a logia body and a seastone Jutte has enough quality to make it as a vice-admiral even without haki.