The first peacekeepers have already deployed
He told the BBC that security remained a problem for the interim administration and there was a need for it to assert itself.
Mr Vendrell said the interim government needed to exert its authority beyond Kabul so that it was accepted by the entire country.
His comments came as reports surfaced of recent fighting between local Afghan commanders in the north of the country.
Afghans' wish
Mr Vendrell said there was a need to disarm the various armed groups with a view to eventual demobilisation.
Asked about reports that rival factions had started preparing to fight in the south and were already doing so in the north of the country, he said that many Afghans wanted foreign troops to arrive.
"For that you do need the deployment of an international force. The international force needs to be deployed beyond Kabul and the Afghans want it - even the warlords say they want it."
The envoy - who is due to resign at the end of the month - said that if a larger international force was sent to Afghanistan, it should have a forceful mandate so that the peacekeepers could defend themselves and respond if attacked.
He said that a force of 30,000 men might be needed. Current plans allow for 4-5,000 foreign peacekeepers to be deployed in Afghanistan.
The UN's new estimate is six times the number presently scheduled to be in Afghanistan by mid-February.
Only Kabul secure
For now there are just 2,000 troops on the ground.
The BBC's Mike Donkin says they are having no real problems but are fulfilling their tightly limited mandate to secure the capital so Afghanistan's fledgling interim government can wield power.
Kabul is calm and trade is busy, but elsewhere there is more tension.
On the highways bandits hold up travellers and loaded trucks.
And in the other major cities like Herat and Kandahar there are reports of fighting between the rival ethnic factions, although details are sketchy.
On Wednesday, Syed Noorullah - spokesman for Deputy Defence Minister and ethnic Uzbek warlord Abdul Rashid Dostum - told the French news agency AFP that fighting had erupted in recent days around Qala-e-Zal, 60km (37 miles) north-west of the town of Kunduz, but the situation was not serious.
"This is very localised fighting between some small local commanders which will have no impact on the general situation," Mr Noorullah said.
Our correspondent says security is the key if Afghanistan is to be rebuilt and its people to get jobs and a normal life again.
But the interim government is an alliance of convenience, he says, and behind the scenes the old Afghan warlords have no great interest in too much stability. They would lose power and profits.