The offer came despite Tsvangirai decisively beating Mugabe in the first round of the presidential poll in March, and Zanu-PF losing its parliamentary majority. The MDC pulled out of the presidential run-off last month in the face of a campaign of violence against its supporters, more than 100 of whom were killed, some in appalling brutal ways, The Telegraph reported.
The MDC insists it is the rightful winner of the elections and is adamant that it will only accept a transitional authority, with Tsvangirai in control, leading to fresh elections in about two years.
"I guess we knew Zanu-PF was not serious and we have never expected anything from South African President Thabo Mbeki," said an MDC source in Harare.
Mbeki, who is mediating the negotiations, wanted to see a government of national unity emerge from the process.
Two officials said that the Zanu-PF negotiators, Justice Minister Patrick Chinamasa and Social Welfare Minister Nicholas Goche, were flying back to Harare from Pretoria, where the negotiations were being held.
Perhaps the only surprise is that the talks, which were scheduled to last for two weeks, got into difficulties so soon after they started last Thursday.
The agreement last week on a memorandum of understanding about the process triggered a wave of optimism in some quarters.
Mugabe has been in power for 28 years and has shown no sign of being willing to give it up, while Zanu-PF is in full control of the country and faces little practical pressure, particularly after Russia and China vetoed a UN Security Council resolution that would have imposed sanctions on the regime.
Zimbabwe already has two vice-presidents, both appointed by Mugabe - one reserved for a senior Zanu-PF figure and the second for a member of the former opposition, the Zimbabwe African People's Union (Zapu) that was consumed by Zanu in the Unity Accord of 1987.