The announcements came on the sidelines of the APEC summit in Papua New Guinea last week.
Tonga's memorandum of understanding on Belt and Road was followed by a reprieve of the country's debt payments owed to China.
Lopeti Senituli, a political adviser to Tongan Prime Minister 'Akilisi Pohiva, told Reuters the loan payments had been deferred for five years.
Meanwhile, the Vanuatu Daily Post reports Vanuatu signed seven different MoUs with China earlier this month, including a Belt and Road agreement.
Chinese President Xi Jinping also approved Vanuatu's request to open a consulate general in China.
Tonga and Vanuatu join the ranks of Pacific Belt and Road partners Papua New Guinea, Samoa, Fiji, Niue and the Cook Islands.
The Belt and Road Initiative, also known as the One Belt One Road or the Silk Road Economic Belt and the 21st-century Maritime Silk Road, is a development strategy adopted by the Chinese government involving infrastructure development and investments in countries in Europe, Asia, Africa and the Pacific.