I don't know of a cake pan that's shaped like a vase. But, you can find a pic of the type of vase you would like to mimic, enlarge it to the size you would like yr cake to be, then print. Cut out the image and use it as a guide up against yr stacked round cakes and began to carve the shape. Do this on both sides of yr cake. Be sure to make yr cake rounds big/wide enough to allow you to carve and still have a good amount of cake left to cover
after you get the basic shape then began to slowly round off the entire circumfrence of the cake shape and then ice as usual.
I hope i explained this well enough for you to understand