The Men in Black played their last scheduled match at the football ground on Sunday ahead of the club's much-anticipated move to the new Allianz Park stadium in Barnet.
Sarries beat Edinburgh 40-7 to ensure progression to the knockout stages and set up a home tie in the last eight.
Heineken Cup rules however state that a team must have at least a 15,000 capacity venue to host a quarter-final match - 5,000 more than can currently be seated at Allianz Park.
Sarries CEO Edward Griffiths said: "We are looking at a whole range of options. We have planned but now some of those plans have got to take shape."
When asked whether Vicarage Road was one of those options Griffiths said: "The options include every stadium in the world. We're just looking at everything available."
Saracens have played at a number of locations this season and are more likely to revisit the likes of Wembley, Twickenham or Stadium MK for the clash.
But Griffiths joked it wouldn't be the first time the club had booked an unexpected reunion with Watford.
He said: "All I'll say is that we played our last game at Vicarage Road but we have said that before - this is our third last game at Vicarage Road."
The match against Ulster will take place on the weekend of April 6.
Saracens host their first game at Allianz Park on Sunday against Cardiff Blues in the LV=Cup.