Even if Vietnam doesn’t offer any investor visas or retirement visas, like Thailand and Malaysia, it’s easy to stay on a tourist visa long-term. Foreigners also have the option to apply for business visas that are valid for 3 months (12 months for Americans).
Still, a preferred choice can be to use the unit(s) as investment vehicles to earn rental incomes. Many new projects and condotels allow you to stay in your unit, or other units in that project, for a couple of weeks every year.