Croatia has made great progress, it has closed 25 out of the 35 policy chapters, and tough negotiations are ahead on the remaining negotiating areas, Vandoren said.
The diplomat pointed to the complexity of the negotiations on the policy chapter "Judiciary and Fundamental Rights".
It is necessary to show progress in the reform of the judiciary, in the struggle against corruption and organised crime, and in the respect for human rights, he added.
According to him, Croatia has to address the legacy of the war such as the issue of refugees, war crimes and the full cooperation with the Hague-based International Criminal Tribunal for the former Yugoslavia (ICTY).
On Tuesday, the EC published progress reports on EU hopefuls.
The report on Croatia commends the country for improving bilateral relations in the region.
Croatia has improved its relations with Slovenia and Serbia, Vandoren said, adding that further progress was expected from a visit by Serbian President Boris Tadic to Zagreb later this month.
Vandoren expressed hope that this would be the last progress report before Croatia's entry into the EU.
Asked by the press about the exact date for Croatia's admission to the EU, the EU ambassador said that the pace of the final negotiations depended on Croatia's ability to convince the 27 EU members and the European Commission that it is ready for entry.
In response to claims by former Croatian Prime Minister Ivo Sanader that he was a victim of politically-motivated persecution in Croatia, about which he allegedly wrote to the European Commission and EU ambassadors in Zagreb, Vandoren said that he had not seen such letters, neither did the Commission confirmed the receipt.
I have heard that he has sent the letter to (Commission President Jose Manuel) Barroso, but Brussels has not confirmed receipt. I can quote prime Minister Jadranka Kosor's words that nobody can be above the law and that state institutions must do their job, Vandoren said.