Well, I like the looks of the St. Crispin's Baby Calf. As far as I know it is 100% veg--a dyed crust. I think it is a bit thin for men's shoes. I've spoken to Tony about it and he showed me some samples of a thicker product but when I went to order, it wasn't available. One of our members spent here on SF some time with Janne Melkerssohn and they had trouble with it tearing.
But it is not the only vegetable tanned upper leather I have seen or used. That said, there isn't much available in the US. Whenever I get a hide that is reported to be 100% veg, I burn a snippet. If the ash is 100% black and crusty, I am satisfied that it is as advertised. If it is a retan there will be turquoise ash in the residue. If it is pure chrome the ash will be an intense greenish blue.
I'd use more if I could get it. Hell, if I has a proper drum dying set up nearby, I'd buy up heavy (3-4 oz.) veg kips and dye them bespoke.
And having said that, I don't mind retans really. I use a lot of buffalo calf in one form or another and it's almost all retan (can't think of any that wasn't). I like it. Done properly, I don't really see a downside.
Speaking of veg tanned leathers and dubbin,oils etc., I use a vegetable tanned lining leather (many high end makers do) and it never gets conditioned with anything. Yet it never seems to crack or dry out either.
I can't speak to whether chrome could be produced as a crust. Of course it could, because all that really means is that it has not been finished. But it would look like hell. And having said that I've seen shoes that were purportedly crust leather (to be dyed and antiqued, etc,) that looks pretty white/grey for even a retan.
edited for punctuation and clarity
Leather goes through curing, beamhouse/preparing, tanning, dressing, and the finishing.
In the tanning process there's veg tan, chrome tan, combination, or chrome tan veg retan. Box/willow calf are traditionally chrome tanned and most thought after.
A lot of those ugly wet blue trace of chrome tan is washed away during dressing, where chemicals are added for suppleness and color.