You can make this any size that you want, just make sure that the wood that you choose for the support is at least an inch wider than the top rim of your vase in all directions.
This piece of base molding is going to act as the back for the wall mount. Now I need to cut my vase support board, I'm using a 1x6 cut to the same length as the base molding.
Use a hole saw to make cut outs for the vases. The holes should be wide enough for the vases to fit through, but small enough so that the top catches and sits on the wood.
Connect the two pieces with wood glue and nails or screws so that the support board rests flat on your work surface and the base molding is behind.
I'm going to add a piece of molding to the bottom of this piece to act as both support and decoration.
Sand any rough edges on the piece so it's nice and smooth, then prime and paint the piece to marry all of your elements together.
Add your vases and some flowers and you've got yourself a great display, I'm Mrs. Fixit and it's just that simple!