"John Garang will be sworn in immediately after 20 February," rebel spokesman Samson Kwaje said in Nairobi on Wednesday.
He was citing the details of a protocol, signed last Friday between SPLM and the Sudanese government, on the implementation of the final peace deal.
According to the power-sharing deal agreed to in May, the leader of the SPLM will occupy the vice-presidency up to the end of the third year, when general elections will be held.
The Sudanese first vice-presidency is currently occupied by Ali Usman Taha, who had been negotiating face-to-face with Garang, since September 2003.
The fate of Taha remains unclear.
Sudan's President Umar al-Bashir and Garang will sign the final peace deal on Sunday in the Kenyan capital, before several African leaders.
"The first thing after signing will be passing of an interim constitution," Lazaro Sumbeiywo, a Kenyan army general mediating in the peace talks, said.
According to the deal signed last week, an interim constitution must be passed by the end of the sixth week from when a final agreement is signed on Sunday.
Also on Sunday, the first half of the oil revenue is expected to be transferred from the Khartoum government to SPLM in line with the wealth-sharing agreement agreed to in January last year, spokesman Kwaje added.
The signature is to definitively end war in southern Sudan, which erupted in September 1983 when the rebels rose up against Khartoum's Arab and Muslim domination of the south. The conflict was complicated by issues of oil, ethnicity and ideology.
The war and its effects have killed at least 1.5 million people and displaced four million others.
The peace deal does not cover a conflict in the western Sudanese region of Darfur, where tens of thousands of people and about 1.6 million others displaced by nearly two years of clashes between rebels and government troops and their proxy militias.
"We will be one rank, the government forces and the armed movement - SPLM"
Bakri Hassan Salih,
Sudanese defence minister
While visiting Eritrea on Wednesday to hold talks with President Isaias Afeworki, Garang pledged to push for a peaceful resolution of the conflicts in Sudan, including those raging in Darfur.
"We will contact with all forces - armed and political parties - to achieve a fair and just and comprehensive settlement," Garang said in Asmara.
There is "no meaning of peace in the south while war continues in west or east or every part in our country", the SPLM leader added.
Choice of army
The peace deal ending 21 years of civil war prompted Sudan's defence minister to offer his southern forces a choice between staying with the northern army or joining a new southern army.
Defence Minister Bakri Hassan Salih gave 369 officers from a variety of southern tribes the chance to join the army of the south, which will be made up of SPLM forces.
"There will be some changes in the coming days because of the peace process," he told the cheering officers. "You will have the opportunity to stay with us or go with the southern forces."
But he added: "We will be one rank, the government forces and the armed movement - SPLM."
The army says it has about 40,000 southern troops under the 369 commanders.
Each officer will decide which army to join during talks after the deal is signed. The troops under his command will go with him.
The SPLM alienated many southerners during the civil war, who accused them of attacking civilian populations, stealing humanitarian aid or favouring some tribes over others.