The US taxes its taxpayers on their worldwide income. So, yes, you are expected to report your income in Vietnam, even if you are not eligible for the foreign earned income exclusion. On the other hand, the US offers tax credits for foreign taxes paid on income reported, so you may be able to deduct some or all of the taxes you paid in Vietnam. Also, if you report this as self-employment income on your US return, you may be able to deduct related business expenses of earning this income.
Oh, and yes, all US taxpayers who have any taxable income are expected to file, even if you don't owe income tax once you have filed. This can actually end up helping you in certain situations, because it documents your earnings.
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