prank of being vietnamese?

This morning, I went to class as usual. Sitting at the back of lecture hall, I was then joined by two other friends. The lecture bored me to hell as usual. Thus, I normally start talking away with those in proximity.

Out of sudden,(while looking in my direction) the lecturer said: The guy... in green shirt, can you understand me?? Apparently he was directing it to me.

Me: yea..following. (Speaking of which, he was teaching LP (not lampa lar...Linear programming okeyyy??)

Him: Good. By the way, are you from Vietnam?

Me: (Thought I heard it wrongly)...nah...I'm Malaysian. (felt insulted)

Him: Oooo...hahahaha (He can always do this kind of successful laughter, as in he laughs but the others don't)

As a result of that (the stupid question), the two friends started making fun of me being a Vietnamese.

Previously, I committed the same crime when I unintentionally regarded "somebody" from India while in fact, he's from Kansas, a nation in the State. I thought it's no big deal, not until today lor...

But yea, js checked the fb and came to realise that this is April 1...So, it could be a prank, right?