IMPORTANT:
I recommend doing this as an experience to see how riding trains is in asia but i wouldnt recommend doing this regularily.
The trains in the first class are dirty, you can hear other people and vietnamese people sometimes tend to watch loud videos or sound music in the sleeping cabins. It is DEFINITELY not like first class in western countries, more like 3 third class travel for europeans.
Ordering tickets with 12go works easy and fast, got an e-ticket to show on the mobile phone at the train station, paid by ViSA credit card. The train was perfectly on time, but any comfort or cleanliness are more or less missing.
We are a family of four and were lucky to get a soft sleeper cabin to ourselves. The cabins were clean and we got some sleep even if the train ride is not that smooth. The train attendant even woke us up 20mins before our stop. The train left and arrived within two minutes of the schedule! The toilets are the only negative. Bring your own toilet paper and don't expect a clean toilet.
The seats are okay and comfortable for an 8 hours drive. Our journey took a bit longer than expected. BUT it was freezing cold during the whole journey. Even though we've expected that and were prepared with jackets and long trousers, this was not enough. Bring a facial mask with you to not get infected.