[LightDestory]

After you disable the networkmanager services with systemctl the network manager will not start up after a reboot. Hence why you use "systemctl disable NetworkManager". This systemctl command deletes all links to the autostart of the NetworkManager services. Til today I have never seen or used "systemctl mask" and I guess I will continue not using it. Given that NetworkManager is simply useless and incompatible with OpenVZ VPSs you should just really uninstall it and be done with it.

Is there something I'm missing? All this fuss hassle about a small issues!?!?!?

The main difference between "disable" and "mask" is that:

1. By using "systemctl disable name.service",as you said too, you will delete the symlink located in "ect/systemd/system" that points to the Unit file located in "lib/systemd/system". With this you will prevent the service to be loaded at boot-time. But the unit file is still there and it can be requested and loaded by other services if they need it. If that happens, you are done. You will kill the network settings again.
   
2. By using "systemctl mask name.service" the Unit file will be linked to "dev/null". This makes impossible for the service to be requested and loaded. So you will not risk an inadvertent loading of that service.
   

When I tried to mask the service I got some "systemctl timeout errors". so as you said, I decided to uninstall it.

This small issue caused by mine distraction requested the support of @deanhills because I was basically cut off, I was unable to do anything. Moreover it ended up on reinstalling from scratch the OS. I don't want to create troubles to anyone and when I had to open a support ticket I felt really bad.

So again one has to keep it simple. Or if you need to experiment more it's much better to get a VPS with a KVM virtualization.

I am loving my VPS 9! OpenVZ is the main cons, but its specs rocks!
The only concern is what VirMach will do after November 2019, let's see and hope!