Question
QUESTION: What else can I use instead of filling the whole vase with floral foam?  Is there any other way to keep tall stems steady?  I know I can use shells, rocks, etc, but this is not cost effective for a very tall vase.  What do you suggest?

ANSWER: Using moss covered foam that rests atop, or securing a smaller container of foam atop the vase will work well providing the vase has enough weight to maintain physical balance.

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QUESTION: Thanks for your help;  I guess the only solution for stems such as forsythia, dogwood, etc is to fill the whole vase?

Answer
For tall stems with heft, such as the ones you mentioned, foam isn't necessary.  You can simply cut the stems - make sure no foliage is below the water line, and inset into the vase as it.  If you wanted some stability at the base, just an inch or two of glass beads or something along those lines would be more than fine.  You needn't fill the entire vase.