Prices hovered below a key psychological level of 40,000 dong ($1.92) per kg for most of April, which discouraged farmers from releasing stocks to exporters, traders said.
Tightening supply from Vietnam, the world's largest exporter of robusta variety, could lend support to global coffee prices.
Last Friday, London's July robusta contract gained 2.23 percent to close at $2,019 a ton.
London softs markets were closed on Monday.
"Sales are slowing so the volume this month will fall (from April)," a trader at a foreign company in Ho Chi Minh City said.
Robusta beans stood at 39,500-39,800 dong ($1.90-$1.91) on Tuesday in Dak Lak, Vietnam's key growing province, from last week's range of 38,800-39,300 dong.
"Farmers at present do not have pressure to sell, after their watering period comes to an end," another trader in Ho Chi Minh City said.
With the rainy season returning on schedule to the Central Highlands coffee belt from early this month, farmers will only need small amounts of cash for fertilizer, traders said.
Exporters were seeking to sell Vietnamese robusta grade two, 5 percent black and broken at a discount of $30 a ton to London's July contract, while buyers bid at a discount of $40 a ton, stable from a range of $30-$40 in late April.
The differentials meant the beans stood at $1,979-$1,989 a ton, free-on-board Saigon Port for outright shipment, down from $2,004-$2,014 a ton in late April.