Brussels - Beijing - Hanoi - Beijing - Brussels

Hello,
I am a Belgian citizen and am visiting friends in BJ.
I have been delivered only a single entry visa for China and already used it, now I'm on Chinese soil.
My friends are planning on going to Vietnam for a couple of days.
Could I join them on the trip and then come back to BJ to catch my flight back to Brussels using the 144H visa-free procedure for my second entry in the country?

Thanks,
Dan