To address a previous observation, I have oiled my veg. tan cowhide with olive oil prior to dying (using modern aniline dyes), and gotten very good results. I find that the dye coat is more even after the oiling, but the color shade will be a little different than it would without the oil.
Looking at different leather-working forums, I found that many saddlers use olive oil routinely. With the peculiar additives in modern neatsfoot oil (ie silicone, etc.) an alternative seemed in order. If you eat the stuff, it should be pretty pure.
I have also used the vinegar / iron dye after oiling with the olive oil, particularly on bark tan deer, and sometimes on pigskin. It still worked, even though I did not think it would. Usually I apply that dye first, and then oil just to be sure it will color properly, and evenly.
Just my experience so far......
Jeff