This style was once the style "a la mode" in Vienna at the turn of the 19th century. Then it fell from fashion and left Europe for the warmer climate of Mexico. Today, there are a number of "Vienna" lagers. I especially like Dos Equis, but with the growth of the specialty beer market it may not hold its special place for long.
The Brewers Association (US) http://www.beertown.org/ offers the following style parameters:
"Beers in this category are reddish brown or copper colored. They are medium in body."
"The malt aroma and flavor should have a notable degree of toasted and/or slightly roasted malt character."
"Noble-type hop aromas and flavors should be low or mild."
"Original Gravity (ºPlato): 1.046-1.056 (11.5 -14 ºPlato)
Apparent Extract/Final Gravity (ºPlato): 1.012-1.018 (3-4.5 ºPlato)
Alcohol by Weight (Volume): 3.8-4.3% (4.8-5.4%)
Bitterness (IBU): 22-28
Color SRM (EBC): 10-12 (20-24 EBC)"
The BJCP offers the following style parameters:
3A. Vienna Lager
Aroma: "A light toasted malt aroma may be present. Similar, though less intense than Oktoberfest..."
"Caramel aroma is inappropriate."
Appearance: "Light reddish amber to copper color... Large, off-white, persistent head."
Flavor: "Soft, elegant malt complexity... Some toasted character from the use of Vienna malt."
Mouthfeel: "Medium-light to medium body, with a gentle creaminess."
History: "The original amber lager developed by Anton Dreher shortly after the isolation of lager yeast. Nearly extinct in its area of origin, the style continues in Mexico where it was brought by Santiago Graf and other Austrian immigrant brewers in the late 1800s."
Ingredients: "Vienna malt provides a lightly toasty and complex, melanoidin-rich malt profile... Can use some caramel malts and/or darker malts to add color and sweetness, but caramel malts shouldn’t add significant aroma and flavor and dark malts shouldn’t provide any roasted character."
"Original Gravity: 1.046 – 1.052
Final Gravity: 1.010 – 1.014
International Bittering Units: 18 – 30
Alcohol By Volumn: 4.5 – 5.7%"