Saracens, who were relegated from the Premiership last season, were taking on Ealing in the Trailfinders Challenge Cup pre-season competition as they gear up for the start of the Championship - which begins in March.
Director of Rugby Mark McCall handed chances to a number of young players, but Vunipola was picked to play.
The No8 is expected to be part of England’s squad for the upcoming Six Nations and he got through 80 minutes under the watchful eye of head coach Eddie Jones.
Vunipola, however, could not stop Saracens falling to a shock defeat at the hands of their fellow Championship rivals.
Ealing were trailing 14-13 at the break, but they fought back in the second-half to claim victory.
Saracens scored in the dying moments to close the gap, however in the end they lost 27-26.