Hi Jason,
Although a Pte Ltd company would help to limit your liabilities when something goes wrong, there are certain fixed costs involved in maintaining such a company. You would be required to maintain a set of accounts and have a company secretary, just to name a few.
A sole...
Actually the last time I made spectacles in Shanghai, I could collect it within 2 hours. There was a spectacles 批发市场。Im guessing it wont take as long in Vietnam too..