I make a finish for my native american style flutes, from beeswax and walnut oil. I take one part beeswax and five parts walnut oil and microwave it until liquid, stir it until mixed and let it cool into a solid kind of like shoe polish. When I am ready to apply it, I melt it in the microwave and brush it on my piece with a foam brush. I then let it harden/cool and rub it out with a soft cloth. It leaves a very nice finish and walnut oil will not go rancid.
While i was reading this and typing, I made a sample piece from a scrap of pre-cased veg tan and it softened it very little and darkened it quite a bit. I then treated a piece with straight walnut oil and it darkened it quite a bit more and softened it pretty good.
I am not sure of what your color/suppleness level goal is, but these are a couple of options.
and +1 on what Alamo just said.
Good luck,
Ken