Pence said in a meeting Friday with Venezuelan opposition leaders in Lima, Peru, that "we are with you to see freedom once again".
The move came shortly before President Donald Trump is expected to address the USA on Friday night amid anticipation of a retaliatory strike for an apparent Syrian chemical weapon attack last weekend that killed more than 40 people.
With the White House's encouragement, Maduro has been barred from the summit over his plans to hold a presidential election that the opposition is boycotting and that many foreign governments consider a sham.
The United States on Friday announced that it's earmarking an additional $16 million to aid Venezuelan migrants who are fleeing that country's hunger, violence and economic collapse.
The vice president says Maduro has turned Venezuela into a dictatorship and has brought about "abject misery". As a hemisphere we must increase our support for the well-being of the Venezuelan people and the restoration of their democracy.
The funding from the State Department and USAID will go to the United Nations High Commissioner for Refugees, the U.N. agency responsible for assisting refugees and displaced persons, and is meant to help Venezuelans who have fled to Brazil and Columbia. In addition to Pena Nieto, Pence is also planning to meet in Lima with Canadian Prime Minister Justin Trudeau.More news: Apple co-founder Steve Wozniak leaving Facebook over privacy concerns
More news: China, Vanuatu deny military base plan
More news: Strike on Syria air base kills 14, including Iranian forces
A Pence aide said Pence planned to meet with Pena Nieto early Saturday, with the get-together only being finalized late on Friday. Although Castro never officially confirmed he would attend the summit, there were high expectations he would show to bid farewell to regional leaders as he prepares to step down in a week's time from the presidency.
Mexico's president says "the door is open" for the United States to join a Pacific Rim trade deal that was initially rejected by U.S. President Donald Trump.
The first leader to arrive in the Peruvian capital was Ecuadorean President Lenin Moreno, who returned to home hours later to monitor closely the case of kidnapped journalists, summit organizers told reporters. After the USA pulled out, 11 countries reached a revised deal last month that dropped some clauses that had been meant to benefit the United States. There was a vision until 2005: it was called the Free Trade Area of the Americas.
Recently, U.S. President Donald Trump directed the deployment of National Guard troops to the U.S. -Mexico border to combat illegal migration and drug smuggling.
In a series of meetings with Latin American leaders, Pence plans to promote good governance and democratic institutions and urge allies to maintain pressure on Maduro.
A copy of the letter was provided to The Associated Press.