A Vienna lager should be 100% Vienna malt, in my opinion!
I made Bradsul's version:
https://www.homebrewtalk.com/f59/brads-vienna-lager-80356/ the last time I made one. It's wonderful!
If BJCP judges gig a Vienna lager made with Vienna malt, then they should be kicked in the pants!
Description:
http://www.bjcp.org/2008styles/style03.html#1a
American versions can be a bit stronger, drier and more bitter, while European versions tend to be sweeter. Many Mexican amber and dark lagers used to be more authentic, but unfortunately are now more like sweet, adjunct-laden American Dark Lagers.
Profile: Light to medium body. Toasty flavor and aroma characteristic of vienna malt. Low to medium noble hops aroma and flavor. Reddish amber to light brown color. No ester or fruitiness. Very low diacytl.
Ingredients: Vienna malt provides a lightly toasty and complex, melanoidin-rich malt profile. As with Oktoberfests, only the finest quality malt should be used, along with Continental hops (preferably noble varieties). Moderately hard, carbonate-rich water.
I LOVE Vienna lager! A double decoction will give all the rich malt flavor needed, and it won't be too sweet. I'd highly recommend making a Vienna lager with 100% Vienna (carafoam also, if you must, but it's not needed) and noble hops. There is nothing like it!