Sharpe, a 67-cap veteran with the Wallabies, was a notable omission from the original 28-man squad for the two Tests in South Africa but has now been summoned by coach Robbie Deans to replace Vickerman, who injured his shoulder in the 27-15 win over the Springboks on Saturday.
"We've been in contact with Nathan and he'll come across," Deans said.
"I'm not sure what the flight details are yet but hopefully he'll get here as soon as he can."
The Wallabies will also be sweating on the availability of Rocky Elsom.
The flanker faces a disciplinary hearing in Durban on Monday after being cited for pulling down Springbok skipper Victor Matfield in the line-out.
Berrick Barnes will miss Saturday's match with a shoulder injury, with Deans indicating a replacement will be found from within the squad already in South Africa.