Hello, I would like to send a bicycle in a box weight 25kgs from Guilin to Hanoi, Vietnam?

Jack, the upper limit of parcels sending to Vietnam is 20kg. I'm afraid you can't send your bicycle through surface mail.
Maybe you can remove some components and pack them in another package. The fare for a 20kg is around CNY150 via surface mail.
If they cannot be separated, you may consider delivering it as a cargo.

Thanks Ella for your reply.

What do you mean by "cargo"? Does it refer to air mail or shipping?
And what would the potential cost and time frame of delivery be?

Thank you.

That means sending through export procedure. For your reference, it should cost around USD100 for a 20GP container and take around 10~15 days.