The world's biggest coffee chain is rolling out a $1 reusable plastic cup starting Thu. Starbucks (SBUX) already gives customers a dime discount each time they bring in reusable cups for refills. As with other reusable cups, the new ones will be boiled clean before refills. Separately, Starbucks said it will open its first store in Vietnam in Feb., just months after its 1st stores opened in India. The store in Ho Chi Minh City will be operated by Hong Kong Maxim's Group, a restaurant chain with holdings in the region. Starbucks shares rose 0.7% to 55.37.