If, as you have written: " Having 6 individual molds is just to big and unwieldy I've found." is true, you will really learn what unwieldy is when you handle a mold for three vases. If you intend to reuse the mold multiple times, as quickly as possible, you need to make the plaster between the vases at least as thick as the outside walls of the mold or there will be moisture problems. Interior walls will take longer to dry, and that means any casting will take longer to set up because the plaster will be absorbing moisture from both sides.
Most working molds need walls at least 1 1/2" thick (refer back to comments made by bciskepottery) and double that to 3" between what you cast . Add up the depth of plaster needed, plus the size of vase (I've allowed 2.5") and you come up with a length of 16 1/2 " , width of 5 1/2", height of 5"; plus the base plaster which also needs to be at least 1 1/2" thick. Actually, if the base is to be 16 1/2" long, I'd make it reinforced with burlap, or mesh of some sort and another 1 1/2" of plaster. Hope you are strong, in good shape, and have a friend to help. Because when you fill the mold and have to dump the excess slip, I'm not sure the word unwieldy will any longer suffice.