The 24-year-old British and Irish Lions tour certainty had been out since November with a knee injury, but he showed no ill effects in a 72-minute performance in Saracens' 35-27 win over the Newcastle Falcons on Monday (NZ Time)
Vunipola's display had English media outlets speculating that he could be recalled to Eddie Jones' England squad for next Sunday's Six Nations game against Scotland.
Saracens built up a big lead at Newcastle before the Falcons rallied with two late tries before stalling in their bid to win two bonus points.
Former Bay of Plenty and Tonga backrower Nili Latu was at No 8 for Newcastle and former Crusaders wing Sean Maitland stood out for Saracens.
Maitland, a Scotland internationalist, could find himself opposing Saracens clubmates Billy and Mako Vunipola and Maro Itoje in the Six Nations arena.
Former Auckland and Tasman wing Bryce Heem was sinbinned before scoring two tries in Worcester's 41-24 win over bottom team Bristol.
Heem was banished in the 14th minute for a tip tackle on Bristol's Ryan Bevington, but returned to score his first try in the 34th minute.
He added another midway through the second half as Worcester - second last on the table - opened up a seven-point gap on Bristol.
Meanwhile, former Hurricanes Victor Vito and Jason Eaton have helped Stade Rochelle take over at the top of the French Top 14 competition after a 27-21 away win over Luke McAllister's Stade Toulouse.
Ex-Crusaders Tom Taylor and Colin Slade kicked 20 points between them as Pau scored a 25-25 draw with south-west rivals Bayonne.
Clermont lost 26-16 at Castres despite a conversion and two penalties from former Crusaders and Blues pivot Stephen Brett.
Retired All Black Ma'a Nonu received a yellow card in Toulon's 15-5 loss to Brive.
Racing 92 - without Ali Williams and Dan Carter after their recent arrests - lost 19-10 to Grenoble. Ex-All Blacks Joe Rokococko and Chris Masoe were in the Racing starting lineup.
In other Top 14 results, Lyon beat Montpellier 16-3 and Stade Francais were emphatic 32-9 winners over Bordeaux-Begles.
Photo: Getty Images (England No 8 Billy Vunipola may be back for the Six Nations clash with Scotland after proving his fitness for Saracens).