Among its other ventures (film, printing, publishing, consumer goods, stationary, and so on),Phuong Nam operates a chain of 30 bookstores, so they're not a complete newcomer to the market. This store will probably do a lot of business. I don't have current stats, but in the past I have seen evidence that there is a sizable number of people reading ebooks in Vietnam.
Wattpad used to release quarterly metrics reports which tracked who visited the social reading site from which countries on what devices. One thing that always stuck out was Vietnam. This tiny country with 86 million people was usually the second largest group of visitors among those who read on a computer (following the US). They easily beat out the UK, Canada, and everyone else.
I should note that Wattpad stopped sharing the metrics report in early 2011, so this data is old. But it is likely still mostly true and it points at a potential customer base.The trick is in getting people to switch to a new device.
Assuming the prices are low enough that the average Vietnamese consumer can afford to buy, this store will be mobbed.