The 29-year-old was taken in by the Ho Chi Minh City police on Sunday, but they have not provided any further details.
Khanh, from the Mekong Delta province of Ben Tre, owns the K-Supper dealership, which has the reputation of selling the most luxurious cars in Vietnam.
He entered the business in 2017 and set up the showroom in HCMC’s District 1, according to local media.
On social media, he regularly posts photos of himself standing next to Rolls-Royce, McLaren, Lamborghini, Ferrari, and Porsche cars.
He also makes posts on how young people can start a business and make money.
His private car collection includes a Lamborghini Aventador SVJ and a McLaren 720S Spider among others.
He is often seen hobnobbing with celebrities.
Before he started selling cars he was an amateur volleyball player until an injury put paid to that.