In a statement, Malacañang said Duterte would "engage leaders" of Asean-member states and dialogue partners, which include China, Russia and the United States, on Manila's position on various issues such as our maritime dispute and illegal drugs.
It was not clear if Li and Pence will hold separate talks on the sidelines of the Singapore meetings, which would be a prelude to a summit scheduled between Trump and Chinese President Xi Jinping at the end of the month in Buenos Aires.
"We must therefore work towards the swift conclusion of RCEP negotiations", the President said in his intervention during a working dinner with Asean leaders on Tuesday night.
A China-backed bid to complete the world's largest trade deal - without the United States - was pushed back to next year after Asia-Pacific trade ministers failed to agree key terms at a Singapore summit.
Addressing a business forum ahead of this week's main meetings, Singapore Prime Minister Lee Hsien Loong called for Southeast Asian companies to invest more in each others' markets and be more open to foreign competition.
The East Asia Summit is a premier leaders-led forum in the Indo-Pacific Region comprising of the 10 ASEAN member states (Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, Singapore, Thailand, the Philippines and Vietnam) and its eight dialogue partners - India, China, Japan, South Korea, Australia, New Zealand, the U.S. and Russian Federation.
On the margins of these Summits, PM Modi will hold bilateral meetings with other leaders.
China is pushing the RCEP deal - Assistant Foreign Minister Chen Xiaodong told reporters on Thursday it "will be of great significance for deepening regional cooperation, coping with unilateralism and protectionism, and promoting an open, inclusive and rules-based global trading system".
The deal was given extra impetus after Trump pulled the USA out of the rival Trans-Pacific Partnership (TPP).
"Free trade has, in some aspects, prevented war effectively", he said in a speech on Tuesday.
"China has opened its door to the world; we will never close it but open it even wider", Li said in the article, in which he called for an "open world economy" in the face of "rising protectionism and unilateralism".
While in Singapore, Moon also plans to attend regional free trade talks, known as the Regional Comprehensive Economic Partnership, involving the 10 ASEAN nations, South Korea, China, Japan, Australia, India and New Zealand.
"It is China's hope that the COC (code of conduct) consultation will be finished in three years time so that it will contribute to enduring peace and stability in the South China Sea", Li said in a lecture.
On Wednesday, Asean leaders and Chinese President Li Keqiang will meet during the Asean-China Summit to discuss concerns on regional issues and enhance cooperation among countries in the region.