Quality of serviced apartments internet?

I think like anything, it depends from building to building. Some building will probably pay for higher speed than others. I believe they're all wifi so it's for the entire building or per floor if it's a big building. Paying more to increase the entire building/floor might not be feasible. Probably won't let you have your own line coming into your unit.

I would try a 3G/4G SIM card.
There is 20GB/month for 200k.

With me this is often faster than via the router of VNTP although we have a 30Mbit/s access.

I will keep it in mind, thanks.

The wifi at my AirBnB was spotty when I was in Saigon last year so I used my phone as a hotspot and went through the data limit pretty quickly. Not sure how much data I bought. Got it at the airport so probably wasn't that much for about the same price. More reliable than the wifi at the place I was staying but could add up if it's your only source for internet.

One of the submarine fiber optic cables for the international connection is constantly broken. This is probably the main reason for the poor Internet speed.

Although the speed tests on my landline show 30Mbit/s, but my Internet is not really fast.
An example is the photo album at Expat.com.  Via Wifi or RJ45 cable almost no pictures are loaded.  On the other hand over 3G/4G all pictures are loaded relatively fast.
I think the speed test apps don't use realistic data packages in their tests.

The 3G/4G network finaly has also to go through the submarine cable.
But I suspect that the 3G/4G data is given a higher priority in the submarine data transfer.