Ortega said on Monday that his choice of a former banker as partner for elections in November sends a positive signal to investors.
The new vice-presidential candidate is Jaime Morales, a retired banker and businessman who is a dissident member of the right-wing Liberal party. In the 1980s, he was a critic of Ortega's Sandinista government, which nationalised businesses.
But Ortega said his choice of running mate was proof that he had changed.
"There should be no fear that anything will be confiscated here. That moment has passed, this revolution is peaceful," Ortega told foreign reporters.
"We have to send a signal to the poor, but also to the private sector. And what better signal than Morales, who comes from this sector."
Morales represented the US-backed Contra insurgents in peace talks with the Sandinista government after fighting had killed 30,000 people.
Their rivalry was also personal - Ortega took Morales's home after the Sandinistas overthrew the Somoza family in 1979, prompting legal battles between the men.
Ortega still lives in the house and the two have said they have reached a settlement over the property.
Ortega, who has lost the past three presidential elections, faces strong opposition within the left as well as from the right.
Morales will try to allay investor
concern about Sandinista policy
Polls show him in a tight race with Herty Lewites, the former mayor of Managua who broke away from the Sandinistas to run, and with pro-business candidate Eduardo Montealegre, who comes from the country's fractured right.
The Contra insurgency crippled Ortega's government. He lost power in 1990 to Violeta Chamorro, who was favoured by the US, and later lost presidential attempts against Arnoldo Aleman, a right-winger, and Enrique Bolanos, Nicaragua's conservative president, both of whom Morales supported.