So after I installed Win 8.1 and redownloaded CSGO I tested it.
Because Iam paranoid I started with every Client 1 Casual Game for 1 Round and after that I started DM.
5-6Clients at the same time in atleast 3x DM (30Mins +) - not a single VAC Error.
Unluckily my RAM Usage went up to 95% (before i had 70-80%) so i cant test more than 6 Clients at the same time.
Also I dont have more RAM available currently - will order some - or someone knows how i can reduce the RAM usage.
As far as i can tell now, fresh Win 8.1 install, only Drivers like Lan, Audio, Usb3, a pretty old graphic Driver (nvidia) and before going into DM starting for 1 Round Casual, it survived for atleast 3-4 DM.
Would be great if someone could test it with more ram or atleast someone who doenst have the Problem with the high ram usage.