What is the percentage of the Vienna in your grain bill? If it is a small percentage, it shouldn't make too much difference if you only use one pound instead of two.
If you're looking to hit a specific OG, then go ahead. The light DME will be closer in color to the Vienna than the amber.
primary: CDA #2
primary: Evil Twin Red
primary: Mosaic Single Hop Pale
Bottled: Oddsides Citra Pale clone, India Cream Ale, CDA #1, Cascadian Dubbel
On deck: Surly Abrasive clone, Yooper's Pale Ale