Last Asia trip before kids. Vietnam, Japan and Taiwan or China

My girlfriend and I want to do one more far trip before having a kid. We already went to Thailand and will go to South Korea in about a month.

We are European so once we have a kid we would mostly travel in Europe.

Would you recommend visiting Vietnam only, Japan and Taiwan combi or China only?

We usually travel around 15 days because she can't choose her vacation days.

Thank you!