Yet Stephens dropped her own serve in the next game as Vandeweghe reeled off 12 consecutive games to wrap up victory in 56 minutes in the opening round clash at the Porsche Tennis Grand Prix.
Vandeweghe, ranked 16 in the world, recorded her first victory over Stephens on clay, having lost their only previous encounter on the surface in Strasbourg in 2015.
"I really don't like clay, it's my least favourite surface," Vandeweghe, who will meet Laura Siegemund in the next round, said after the match.
"I'll never like it. This is my second time in Germany -- I'm not here for the clay, it's more the shopping. I didn't come out here to play and lose."
Halep came back from a set down against Slovakia's Rybarikova in their second round match.
The 26-year-old Romanian relied on her ferocious groundstrokes to secure a decisive break in the sixth game of the final set and went on to book a spot in the quarter-finals.
Former world number one Angelique Kerber avenged last weekend's Fed Cup defeat to collect a 6-3 6-2 win over Czech Petra Kvitova in a first round match.
The German will meet Anett Kontaveit in the last-16 after the Estonian saved two match points to knock out last year's runner-up Kristina Mladenovic 5-7 7-6(3) 7-6(5).
Russian Anastasia Pavlyuchenkova battled past U.S. Open runner-up Madison Keys 7-6(7) 5-7 6-4. The 26-year-old will take on second-seed Garbine Muguruza in the second round.
(Reporting by Hardik Vyas in Bengaluru; Editing by Toby Davis)
