I have done the vaseline, too. It worked pretty well for a long time. Then, it just stopped helping and I switched to coconut oil. Dry and cracked heels are very common among Diabetes patients...so I figure it is just another fun thing that comes from insulin resistance. Also, mine seems to improve when I keep my carbs lower which reinforces the prediabetes/diabetes connection, IMO.
I have not noticed having dry or cracked heels, although I have terribly dry skin at times and I associate it with the PCOS.
When I'm on BC to control it, the dry skin goes away. Not entirely, but it's a pretty big difference.