Why are there no new catalyst drivers for Windows Vista 64? Vista is still a supported Operating system by Microsoft. Can the lack of new drivers cause instability?
No, Microsoft stopped support for Vista in 2012. AMD have no new drivers. What drivers are available can be found by manually searching using steps 1-5.
Your best bet is to install the Windows 10 PREVIEW now, so you can get a free copy at the release which is the end of July.