England captain Farrell is absent for the Heineken Champions Cup holders after being issued with a five-match ban for a dangerous tackle against Wasps earlier in the month.
It has put Sarries on the back foot even more for the last eight clash, which is a repeat of the 2019 final.
Farrell was instrumental in the victory last year but will have a watching front at the Aviva Stadium and it means either Alex Goode or Vunipola, the cousin of England internationals Billy and Mako, will play at 10.
Disciplinary | Owen Farrell receives five-match suspension.
More ⬇️— England Rugby (@EnglandRugby) September 8, 2020
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“We have two choices,” McCall said. “To move Goodey up and play him at fly-half and Elliott (Daly) to full-back or stick Manu in there and keep Goodey where he is. We will make that decision later on.”
It has been a tumultuous season for Saracens, with the club handed a hefty fine and docked 35 points in November for repeated salary cap breaches before they were automatically relegated from the Gallagher Premiership in January.
A rare bright spot has been the development of fly-half Vunipola, who starred to help a young XV beat an equally inexperienced Exeter on Sunday.
He also caught the eye of his director of rugby when he kicked Saracens to victory at Gloucester in November, days after the double-winners had first been punished for several salary cap breaches.
McCall added: “Manu has certainly shown all year any time we’ve given him a new challenge, he has stood up to that challenge.
“One that springs to mind the most is the game down at Kingsholm. It was a few days after the original decision and quite an emotional occasion.
“As a young player, he handled that. It is not the easiest place to play as a young fly-half and he handled himself very well that day. All season he has continued to gain these experiences.
“He has played Racing 92 away, Munster away and he was involved in the Ospreys game. He has accumulated a lot of experience this year and we have a lot of faith in Manu.”
Saracens are aware of the “massive challenge” they face in Dublin against a team who have not tasted defeat since these sides met in Newcastle in May 2019.
He has played Racing 92 away, Munster away and he was involved in the Ospreys game. He has accumulated a lot of experience this year and we have a lot of faith in Manu.
Leinster prepared for this fixture with success over Ulster in the Guinness PRO14 final on Saturday while the English champions have been able to rotate their squad in recent weeks with nothing to play for in the league.
This weekend could be Sarries’ last in Europe for a long time and McCall says the competition has been a lifeline for them after they triumphed in must-win clashes against Ospreys and Racing to qualify for the knockout stage.
“When we got the 35-point deduction in early November, we quickly made the decision to put all the eggs in the basket of Premiership survival,” he said.
7️⃣x🏆
Check out the match gallery from our #GuinnessPRO14 Final win at the weekend: https://t.co/IcPdVn7LFH 📷#GuinnessPRO14 #LEIvULS pic.twitter.com/bDsnT7UKus— Leinster Rugby (@leinsterrugby) September 14, 2020
“That meant conceding Europe for that season and we continued that strategy I guess until the penultimate game, which was up in Swansea.
“We actually heard on the Friday night, I got a heads-up it would be automatic (relegation), and we were playing the Ospreys the next day.
“We had left most of our experienced players at home on the basis of the original strategy and Rhys Carre got sent off in the first five minutes, so that win that day with a really young group and some really good senior players just gave us the lifeline we needed.”
