England head coach Eddie Jones watched from the stands ahead of their opener against Scotland on 6 February.
Sarries are playing in the Trailfinders Challenge Cup to be ready for the start of the Championship season on 6 March.
They were relegated last season because of persistent salary cap breaches.
"I thought Billy was outstanding, genuinely. He has been brilliant with this young group all week. He has been really positive with them," McCall said.
"It's so hard for an international player to play in a game like this and not get frustrated at times - with your team-mates and with the opposition. But I'm really pleased."
It was Vunipola's first game for six weeks, in a starting XV featuring six internationals, though none of his England team-mates featured as they are all on individual training programmes to get them ready for the Six Nations.
The Challenge Cup is a pre-season tournament featuring Ealing, Doncaster and Sarries, who were without eight players suspended for breaking coronavirus rules when on Barbarians duty.
"Billy is just a great club man, he knows the importance of senior players and their influence on the younger players," added McCall.
"Some of these younger players are just starting their journey and to have someone like Billy to guide them through, stay positive and be supportive, is very good."