Speaking at the University of Bucharest’s Central Library, which was heavily damaged by fire during Romania's anticommunist revolution in December 1989, Biden called the event part of "an extraordinary season of change."
"Twenty years ago the world watched in awe and admiration as the men and women throughout this region broke the shackles of oppression and emerged a free people," Biden said.
Biden said the collapse of communism had inspired the world, and 20 years on, he urged the countries of Central and Eastern Europe to help countries further east still struggling to firmly establish flourishing democracies.
"You can help guide Moldova, Georgia, and Ukraine along the path to stability and prosperity," he said, adding there was also "much work to be done in Armenia, Azerbaijan, and Belarus."
Biden's trip is designed to reassure U.S. allies Poland, Romania, and the Czech Republic that the United States' commitment to the region remains strong.
On missile defense, Biden said in his speech that President Barack Obama had scrapped Bush-era plans in favor of a system he said would be better and more flexible.
'Nothing About You Without You'
Russia has welcomed Washington's decision to revamp the original plan to put missile interceptors in Poland and a radar in the Czech Republic.
But the move caused concern in Prague and Warsaw that their interests might be overlooked as Washington pursues better relations with Moscow.
In his speech, Biden said some had jumped to the conclusion the decision "was meant to appease Russia at the expense of Central and Eastern Europe."
"Nothing could be further than the truth," he said.
Biden said better relations with Russia would benefit all, and that Washington and Moscow shared some common interests, including stabilizing Afghanistan and preventing Iran from acquiring nuclear weapons.
But he said Washington was not naive, and continued to disagree with Moscow on basic principles.
Biden said the United States stands against the 19th-century notion of spheres of influence, and supports the right of sovereign democracies to choose their own alliances.
Washington, he said, would never make a deal about anything "above your heads or behind your backs."
To stress his point, he twice repeated, "Nothing about you without you."
Reaching Out To Moldova
Biden touched on the aspirations of one fledgling democracy in particular, Moldova, earlier in the day at a news conference with Romanian President Traian Basescu.
"We share a desire, as well, that Romania's neighbors, including Moldova, will continue along the path towards democratization and economic reform and that they will be integrated into the Euro-Atlantic institutions when they are ready," Biden said.
"And that is why, as you pointed out, we are participating in helping, hopefully, stabilize economically Moldova."
During his trip so far, Biden has received support from Bucharest and Warsaw for Washington's revamped missile-shield plan.
The new proposal calls for a more mobile system targeting short- and medium-range missiles, first with sea-based interceptors and later with land-based systems.
Biden heads to Prague later on October 22 for the final stop on his tour.