Windows vista will technically run on that system, however without support for Intel USB 3, you will be limited as to what can be done with it. IF someone were to make a signed driver supporting Intel USB 3 then it would work in a very basic limited sense as most Intel chipset drivers no longer support windows versions less than 7. Nicely enough, that 980TI IS supported on Vista!
as an "asides" windows vista seems to be quite unstable on Intel Haswell chipsets so i can only imagine it is the same or worse on Skylake.