I imagine that MAGIX are not specifically stating support for any Windows version other than W10 given that Microsoft has long since ceased Mainstream Support for both W7SP1 and W8.1, only security updates being offered and, in the case of W7, even that ceases on 14 January next year. More info re Windows lifecycle can be found at:
I can understand why... if there's some underlying issue in windows that is causing a problem in Vegas, there's no hope of them fixing it in an update, so no hope of being able to fix the problem it is causing in Vegas.
DirectX 12 API in windows 10 was created to remove some of the CPU bottlenecks by allowing for developers to use low-level programming conventions to shift some of the pressure points away from the CPU. There is now also support for ray tracing (RTX) as in the new NVIDEA line up. High time to move to windows 10 folks!