A couple of ideas to get you started...
Go to Amazon, and type in vietnamese food. How many books come up? If there's a selection of mainstream book titles, there's probably money being spent in the niche.
Go to Magazines.com and do the same thing.
But don't let that be your "go, no go" to get started.
Go to Shopping.com, scroll down to the bottom of the main page, and click on the link in the bottom set of links that says "top searches"...are people looking for things related to the niche you want to market in (hint: click on "Food and Drink" first in the list that comes up)?
I'm giving away one here, but click on this link
vietnamese food Food and Drink - Compare Prices and Buy vietnamese food Food and Drink at Shopping.com
That was a search on Shopping.com for "vietnamese food".
Look at the results. Do you see enough there to plan and create an affiliate business around?
Then...and most importantly...
ask people.
Talk to people outside of the 'net about vietnamese food (
not your business unless it's other business people, many people still wonder how you can make money with this "internet thing).
Listen to what they say. See if there could be enough interest.
There's a ton more on "niche discovery", this only scratches the scratch of the surface, but it should get you started with ideas for this niche...or other ones.