According to the housing law of Vietnam, foreigners are allowed to rent out their apartment, BUT they must notify the local housing management agency and of course they have to pay taxes. So yes, you may lease out your property as long as you notify the government and pay taxes.
However, people don't like to pay taxes, so they make up ways to avoid paying the full price when it's possible. For example, I personally know a landlord who made the lessee sign TWO lease contracts: one for the tax department, and the 2nd one with the actual rent price which is 30% higher! Very cunning landlord I must say)))
Anyway, if you're going to rent out an apartment, you better do it in accordance with the law. You certainly don't wanna get caught cheating by the authorities, believe me! Good luck man