Well, like I said...I've never used a vegetable tan. But here's what I know about them.
Usually the bark, tannin blend, gambier, or here in your case, Alfalfa stems and leaves, are boiled to extract the tannins.
Mix up one ounce of salt to each gallon of water that you will be using and set it aside for now. If you used a gallon of water to boil your tanning agents in then add another ounce of salt to your salt water solution now.
After the boiled tanning agents cool, add the solution to the salt water that you have ready.
You'll need to get some PH papers and keep the acidity level of your tanning solution between 4.5 to 5. You can adjust the PH level down by adding very small amounts of dissolved baking soda to it. Chances are, once you have your salt water and boiled tanning agents mixed the Ph level will be somewhere around 3 to 4. You can adjust it down from there. When I say down I mean raise the number level. (by adding the baking soda) The higher the number, the lower the acidity. All vegetable tans need to be 4.5 to 5 to properly tan.
Once you have you PH level adjusted you can add your skins. Agitate the skins in the solution as often as possible. Small light skins will need to soak for approx. 5 days. Larger heavier skins anywhere from 5 to 14 days.
Once the tan has fully penetrated the skins you can remove them, drain them, oil them, and stake them.
Check your skins for full penetration by making a small slice on an outside edge. The color should be uniform all the way through.