Mate, I grew up playing league at school in the late 70's. Around the time of the last refugee influx. They just happened to be Vietnamese then.
We had a kid that was "allocated" to our school in Dundas. Couldn't speak a word of English, but made the First XIII on the wing. Was as fast as that Looney Tunes Road Runner dude. Beep Beep. He ended up being one of our best players and became a school legend. It can happen.
I remember playing against Fairfield Pats in the early 90s and half their backline were Vietnamese. They were only little blokes but didn't lack for footwork or toughness. I remember chasing their fullback in a circle like a f**king simpleton.
In saying that, I don't know many Vietnamese players in the NRL, so to be perfectly honest, I haven't a clue what my point is.
There's nothing stopping a kid of any ethnicity from being a star in junior footy. But at the top level it's heading more and more toward the first requirements of a player being speed and strength. That's why Polynesians make up about 1 in 3 NRL players even their numbers in the wider population aren't even close to that. I'm sure I'm not telling you anything you don't know already.
That said, there are always outliers. There's a Chinese guy in the NBA for example, so I'm sure we'll eventually see an Asian player in the NRL.
And Alex Chan doesn't count.