Not familiar with Europe, but I will talk about America.
I was posted to work in the US many years ago. I lived in Orange County, CA, not far from Little Saigon. There were plenty of restaurants serving Vietnamese food.
But I also came across a few restaurants which served both Chinese and Vietnamese food.
As pointed out in other answers, many of the Vietnamese in the US are of ethnic Hoa (Chinese). It's no surprise that they open restaurants which serve what we call Chinese American food.
(Just like Vietnamese, Chinese from Mainland China are very proud of their cuisines. But Chinese American food is different from what you would find in China.)
There is another explanation: restaurant owners do not always work in the kitchen. Instead, they hire cooks. There are more cooks who are familiar with Chinese American food. After all, there are more ethnic Chinese than ethnic Vietnamese in America, and Chinese American food has far longer history.