Fed Cup: France and US 1-1 in semifinalsSloane Stephens of USA celebrates after defeating Pauline Parmentier of France during the Fed Cup semifinal singles tennis match in Aix-en-Provence, southern France, Saturday, April 21, 2018. (AP Photo/Claude Paris)
The 20th-ranked Mladenovic dropped serve five times but broke No. 16 Vandeweghe's serve six times.
Earlier, U.S. Open champion Sloane Stephens struggled to put away Pauline Parmentier 7-6 (3), 7-5 at the 6,700-capacity Arena Pays d'Aix on indoor clay.
Stephens saved five set points serving at 5-4 down in the first set, and rallied from 4-3 and a break down in the second set against Parmentier.
Both players made more than 50 unforced errors each in the scrappy 2 1/2-hour match, conceding 25 break-point chances between them.
Stephens converted four of her 11 break chances, while Parmentier took only three of her 14 chances.
The Americans hold an 11-2 winning record against the French, who won their most recent tie in 2014.
In the other semifinal, Germany trails the Czech Republic 2-0 in Stuttgart.