With "software" being as layered as it is, I always blame the first layer below (or above, depends on which direction you view it). In this case the layers are roughly Windows (OS) - Civilization IV (Application) - Mod (Application extension).
So, I have to agree with Kael, it's the modder's fault in first instance, but the modder will blame Firaxis for not making easier tools to debug it's mod (although there are enough in CIV to make an error-free mod) and Firaxis might curse Windows for being closed source, which hinders them in writing better code.
One thing I hate about CIV crashes-to-desktop is the lack of feedback. I'm not sure who's to blame there, but one thing I noticed is that error feedback changed from Warlords to BTS. With Warlords I seemed to get more information on why CIV was crashing...
I'm no fan of Vista, but generally I like strictness in software. On the long term it will lead to better programming. As a consumer market OS, I consider Vista to be a failure and Microsoft probably thinks so as well. If you want to upgrade, upgrading from XP to Windows 7 and skipping Vista is likely the sanest thing to do. Personally, I'll stick to XP for the years to come.