France won the dead-rubber as the tie finished 3-2. The US will face the Czech Republic in the finals, held in November 10-11.
But she was unable to repeat those heroics against Stephens who had given the Americans their first point when defeating Parmentier 7-6 (7/3), 7-5 in Saturday's opening match.
"I knew she was going to play well so I was prepared for that", said Keys.
"I played really solid so I'm really pleased with that and I'm happy to get my team up 2-1".
"The Fed Cup has been extremely important for me both as a player and a person", said the 26-year-old New Yorker. Thanks to Keys' stubborn performance, US is bidding to win 7 consecutive Fed Cup ties for the first time since 1989-91, when the team sealed 14 straight wins. So just being able to represent my country is the thing that I'm always really excited about. "Maybe I (would prefer) France, USA you never know they have some great players but any team in the final has great players".
The French pair of Amandine Hesse and Kristina Mladenovic edged Vandeweghe and Bethanie Mattek-Sands 6-4 3-6 10-6.
Captain Kathy Rinaldi, of Orlando, has had a very successful run as U.S. Fed Cup Captain since being named Captain in 2017, now holding a flawless 5-0 record at the post. EPA-EFE/GUILLAUME HORCAJUELOFrench supporters cheer for their team during the Fed Cup World Group semi final tie between France and the United States in Aix-en-Provence, southern France, 21 April 2018.
Now both teams will look forward to the November final when the US squad could potentially pull a Williams sister or two into their line-up. Rinaldi now serves as Lead National Coach, Team USA - Pro Women for USTA Player Development.
Germany have only beaten the Czechs once in eight meetings and that was back in 1987 in Vancouver.
It defeated Belarus 3-2 in the Final in Minsk, with Shelby Rogers/CoCo Vandeweghe defeating Aryna Sabalenka/Aliaksandra Sasnovich in the decisive 5th rubber.