As always, I'm going to assume you are familiar with preparing your fabric, the ingredients we use, and the one color baggie dyeing method. If it's been a while, please take a moment to review these.
Vat dyeing is good for making larger quantities of fabric a fairly uniform color. Why on earth would you want to do that? I find vat dyeing is useful for overdyeing commercial fabrics, for getting a background color prior to painting fabrics or doing special effects, and for dyeing backings or linings for wearables. I also may wind up using it on a natural muslin vest I made recently -- by the time I did the binding, I ran out of the original muslin, and the binding is a slightly different color. I'm considering vat dyeing the whole thing a light taupe to even out the muslin differences.
The recipe that follows is for medium colors -- use more or less dye for darker or lighter colors. Remember, you can always dye it a second time, but it's more difficult to remove dye.
Prepare your fabric as usual. Get out all your usual dyeing supplies, and a bucket big enough to hold your project and still allow room for stirring.
First, you'll need an idea how much fabric you have. Most recipes for vat dyeing give you the amount of dye per pound of fabric, but who wants to weigh the fabric? We'll be adventurous here, and assume that we're using a good weight of PFD muslin, like Kona Cotton. That means about 3 yards of fabric will weigh a pound. (If reproducing your results is critical, take the time to weigh your fabric.)
For each yard of fabric, make the following recipe of chemical water:
1 gallon lukewarm water
1 cup plain salt (optional)
2 tbsp Soda Ash
If you have two yards of fabric, make twice as much; for three yards, make three times as much, etc.
Make sure the soda ash and salt are completely dissolved. This may take a bit of stirring. Take 1 cup of this chemical water, and dissolve 1 teaspoon of dye into it. (If you are using two yards of fabric, dissolve two tsps, etc.). When the dye is completely dissolved, pour the dye into the rest of your chemical water, and stir well.
Put the dye/chemical water mixture into the bucket, and add your fabric. Stir frequently for about an hour -- the more you stir, the more even the color will be.
After an hour, rinse and wash as usual. And remember to put your test strips in your notebook!