As a design engineer's viewpoint, I would answer your question of if Vietnamese engineers are happy.

I would say I am happy without any thinking. To work for Big Guys is a pride. Moreover, to see Big Guys' investment bringing opportunities to young engineers within both career path and education is happy also.

However, I prefer companies setting up design houses in Vietnam rather than manufacturing. Manufacturing considers engineers as indirect labour but design houses treat as direct labour, which bring more happiness to engineers.