I had a try with Windows 7 to see if I could reproduce the issues you experienced. At the end of the Arduino SAMD Beta Boards installation, you are prompted whether you want to install the drivers.
For the first test, I declined the automatic driver installation. I then did a manual driver installation by pointing the Windows driver installation wizard at the drivers subfolder of the Arduino SAMD Beta Boards installation at C:\Users(Your Login name)\AppData\Local\Arduino15\packages\arduino\hardware\samd_beta\1.6.25\drivers. It installed the drivers and I was able to use the my MKR Vidor 4000 board with no problem.
For the second test, I confirmed the automatic driver installation. There were a couple of dialogs where I needed to confirm the installation due to the Windows security settings. After the automatic driver I was able to use the my MKR Vidor 4000 board with no problem.
So from what I can tell, the driver system for MKR Vidor 4000 is working just fine with Windows 7. I also find it encouraging that it's now working fine for you.
max2kow:
I had to manually find the right driver
Are you certain you didn't deny the automatic driver installation when you installed Arduino SAMD Boards?
max2kow:
and it worked but only after I tried several baud rates between the desktop's USB baud rate and the actual one on the MKR board
How did you set the baud rate? Are you talking about setting it via Serial.begin() in your sketch and the baud rate menu in Serial Monitor?