There have been reports of increasing lawlessness in Kabul
Speaking in an interview with the BBC's Pashto service, Mr Vendrell said that while Mr Rabbani remained head of what he described as the Islamic State of Afghanistan, that did not necessarily mean he would be the president of the new Afghanistan.
Although the United Nations still recognises Mr Rabbani as the country's president, many Afghan groups reject his authority.
And fears are growing that the power vacuum left by the Taleban's removal will plunge Afghanistan into a chaotic power struggle among warlords and rival ethnic groupings.
Mr Rabbani is reported to be due to return soon to Kabul, which has now been taken over by one faction of the Northern Alliance.
He has been expected in the capital since Wednesday, but it remains unclear why he has still failed to appear.
Mr Vendrell is also heading for the Afghan capital on Saturday to co-ordinate talks on forming an interim broad-based government.
UN pressure
A meeting between the UN and the leaders of various Afghan groups is expected to take place next week outside Kabul - although the Northern Alliance have been pushing for the meeting to take place in the capital.
The UN special representative for Afghanistan, Lakhdar Brahimi, has called on Afghan factions to agree quickly on a time and venue, warning that any delay would make the process increasingly difficult.
"The more time is wasted, more problems may crop up and make progress that more difficult," he said.
With the help of the western coalition and other governments, Mr Vendrell said he hoped to be able to cobble together some kind of interim council to administer Afghanistan until the convening of a grand assembly, or loya jirga.
That loya jirga would then decide on a future provisional government.
Russian view
Mr Vendrell's warning that Mr Rabbani is not assured of the leadership came after Moscow announced it was also sending a delegation to Afghanistan to make contact with the Northern Alliance.
Russia's Defence Minister Sergei Ivanov has described the Northern Alliance as Afghanistan's "legitimate government."
Russia continued to recognise the Northern Alliance as the legitimate administration in Afghanistan throughout the years of Taleban supremacy and gave it military assistance.
The BBC Moscow correspondent says Russia is now signalling to the West its determination to secure a dominant role for the alliance in the post-Taleban leadership.
It is still unclear to what extent the Russian move could put Moscow on a collision course with Washington and other Western countries that want to ensure that the Northern Alliance does not dominate the next government, analysts say.
The coalition against terrorism may pride itself on the relative ease with which the Taleban have been removed from power.
But the fall of Kabul has created a volatile and dangerous situation.
Many mujahideen commanders and tribal leaders do not recognise Northern Alliance authority.
And ethnic rivalries could easily turn elements of the alliance against each other, rekindling the vicious factional fighting which ripped Kabul apart in the early 1990s.
International presence
There are reports of rising lawlessness in the capital.
The BBC Afghanistan correspondent says ordinary Afghans do not trust the militia leaders, and are desperate for an international military presence.
The alliance is dominated by ethnic Tajiks, like Mr Rabbani, who are Sunni Muslims.
Their Jamiat-i-Islami group is now largely in control of Kabul, and has said there is no need for international peacekeepers.
But several hundred ethnic Hazara troops - minority Shia Muslims persecuted by the Taleban and other previous regimes - are heading for the capital to protect Hazaras and to demand a share in any administration.