To fill a really tall vase, I bought some tall flowering branches. I am absolutely in love with how it draws the eye in, and softens the room, and so are all my guests. I have gotten complements from everyone who has been over in the last few days! These flowering branches (there are many different types, think cherry blossoms) are a great diversion from short vase flowers, and really bring nature inside! If you are looking for a more long term solution, Jo Ann Fabrics has some surprisingly real life flowering branches for $2 a stem.
I just can't get enough, they are soooooo pretty!