Both Deeney and Vrancic have been out since the first few weeks of the campaign but could make their returns on Friday night at Carrow Road when the bottom two in the Premier League face off.
Deeney has been absent with a knee injury, which required surgery, but has been back in full training this week and boss Quique Sanchez Flores must decide if the key striker is fit enough to start or be named among the substitutes.
The Hornets have struggled for goals in the captain's absence, registering just six all season and they sit bottom of the table still without a win from 11 games.
Etienne Capoue and Ismaila Sarr have also returned to training and will be assessed ahead of the game, while Craig Cathcart has overcome a back spasm.
Sanchez Flores is pleased to have Deeney back in contention for the first time since he returned to the club as boss in September.
"He's ready to come to the group and go to the list," he said. "He is ready and we will make the final decision. It's very good news."
Norwich midfielder Vrancic has recovered from a calf injury and then illness and is expected to be in Daniel Farke's squad on Friday, while Ibrahim Amadou and Todd Cantwell are now fully fit and could start.
Grant Hanley and Christoph Zimmermann remain out but are set to make their returns to action after the international break that follows this weekend's Premier League action.
Hanley has not played since the defeat to Chelsea on August 24 and has since undergone groin surgery while Zimmermann has had an ankle injury since the end of August.
The Canaries have managed just a point from their last six games and would sink to the bottom if Watford were to secure their first win of the season on Friday.