The British firm said it will buy the stake from fixed-line group Telkom to add to its existing 50 percent stake, as it pushes further into emerging economies.
Telkom and Vodafone had been in talks since June over the group which has strong market positions in South Africa, Democratic Republic of Congo, Lesotho, Mozambique and Tanzania.
Shares in Telkom were up 3.1 percent at 0940 GMT while shares in Vodafone were down 3 percent in a weaker London market.
Vodafone expects the deal to complete in the first half of 2009 and said it would dilute adjusted earnings per share after acquired intangible asset amortisation for three years but be accretive thereafter.
"We will continue to support the management team in their strategy of transforming Vodacom into a full service provider in Africa," Vodafone Chief Executive Vittorio Colao said. "We are confident that the transaction will deliver value to our shareholders."
Telkom says the Vodacom sale would lower headline earnings by 38 percent.
Telkom said earlier this month that its board and the South African government — a shareholder — backed the offer.
Vodacom Group will be listed on the Johannesburg Stock Exchange and the remaining 35 percent of Vodacom Group will be demerged by Telkom to its shareholders.
Telkom said the sale would unlock significant value for Telkom shareholders in volatile markets and it would focus after the deal on its fixed-line business and expansion opportunities. Reuters
As part of the deal, the Vodacom identity will remain visible on the African continent and it will be the exclusive investment vehicle through which Vodafone will make acquisitions in sub-Saharan Africa, excluding Ghana and Kenya where Vodafone is already present.