The key to soft leather, is work, lots of work. You have to continuously work the leather as it dries, or it will be very stiff. Vegetable tanning is what is used for tooling leather, and is "true" tanning, in other words, uses tannen from tree bark. It makes a tough leather and does well with moist environments. The process is long, and can take as long as a year in the bark bath.
Eric