Bean prices in the Central Highlands, Vietnam's main coffee growing region, fell to 34,500-35,000 dong ($1.48-$1.50) per kilogram from 35,600-35,700 dong a week earlier, traders said.
Farmers were not selling much due to low prices and as little stock was left for the current crop year, which ends next month, while exporters struggled with weak demand during the low season of summer and ample output forecast for the next crop.
Last week, traders estimated output for the 2018/2019 crop year would rise 4 percent from the current year to 1.85 million tonnes, or 30.8 million 60-kg bags, which is higher than United States Department of Agriculture's 29.9 million-bag estimate for 2018/2019.
Traders forecast August coffee exports at 100,000-120,000 tonnes, down slightly from a government estimate of 130,000 tonnes exports in July, but both still higher than the same months last year when output was hit by bad weather.
Vietnam's 5 percent black and broken grade 2 robusta was traded at a $80-$110 per tonne discount to the London November contract, compared with last week's $105 discount to the September contract, traders said.
Meanwhile, robusta beans in Asian rival Indonesia's Lampung province were traded at a $50 premium to the November contract, the same as last week, despite increasing supply during the peak harvest season, traders said.
A trader in Lampung said many buyers sought premiums below $50, but "no one wants to sell cheaper" than that. He estimated the high supply volume would remain for the next two weeks as the harvest peaks, before output starts to fall.
Indonesia's coffee output this year is expected to drop 40 percent on "extreme" weather conditions, a local industry group said last week.
Sumatra July robusta beans exports fell 51 percent from last year to 16,306.8 tonnes, but that was a four-fold jump from June as the province is in its harvest season, official data showed.