On a recent Thursday morning in Hanoi, Chinese electric vehicle giant BYD began the day with a showroom devoid of customers. Just a few kilometers away, a dealership for upstart Vietnamese EV maker VinFast Auto was buzzing, with would-be buyers pouring over new models.

"We see an average of 20 customers every day during weekdays,” said Tran Trung Hieu, a VinFast salesman in a black suit and tie. "That can double or triple on a weekend.”

BYD’s entrance in July into Vietnam, whose youthful population is keen to buy electric and hybrid cars, underscores both the opportunities and challenges Chinese companies face as they seek to push into Southeast Asia.