Vietnam post

No, you can't 
But you can send it as parcel and keep the letter inside parcel box!

You will have to complete a VN Customs declaration.

You are best to pack the object in a padded envelope (whatever), complete the addressing, etc. DO NOT SEAL. (The clerk might want to verify your Customs Declaration)

Go to the main Da Lat Post Office (Still at 02 Le Dai Hanh Street. Main Post Office hours are 07:30-21:00H daily)

Take your goodies to the correct desk AND REQUEST THEY BE SENT REGISTERED. The clerk will supply a Customs form. She may use a machine tape for postage OR stamps.

If she uses STAMPS make sure she 'cancels' them with a rubber stamp. This will stop the stamps being stolen and re-sold for cash. Packages without stamps don't leave the country! Nor are they returned!

When you ask 'How long', whatever the clerk says, double it and you will have a better idea when it will arrive.

Many larger Post Offices have packing materials for sale. Also tape the items to thin cardboard to make it impossible to 'shake' put of the package if a corner is cut!  Put 'clear sticky tape' across the package in both directions for added security.

My employer sends numerous small items out each month even as large as robotic remote controls - if you use a 'box', which will likely be small, it will require special handling an automated systems overseas whereas as a larger, flatter, envelope from the P.O. will whiz through the automation.

Jaitch, thank you very much for your answer!

We have sent a parcel like you said.
It was cost 25000 VND by track number. And 47000 VND by letter (about 100 gramm).

Do you know what that meen - "Đóng chuyến thư"?
It's the last track number status at the Vietnam Post website.
Google translate it like "closed trip letter".

Is it normal or not?