Your numbers are a bit off, true that the Vista base is small (5-10% probably), but People tend to buy software when they buy a new PC, not that often halfway in between. About 50% of the PC sold today are sold with Vista (give or take a 10%).
Warning, guestimate follows:
Vista is selling at the rate of 20 million copies per month. Some PC's are still being sold with XP (lets be kind and say 10 million).
Over the past 6 years, at worst, XP has been selling at an average of about 25 million copies per month. That gives us about 1850 million copies to date. Vista could only have sold 100 million copies to date. So at best Vista has about 5% market penentration if you exclude Macs, Linux, all old versions of Windows and the people who have uninstalled Vista for XP.
it sounds stupid to me that a software developer or hardware manufacturer would not put the extra effort in to make their stuff work on Vista.
Sometimes that extra effort is not just a little but a lot. Thats not to say that people are not busy doing it but it takes time. Here are some of the problems:
1) Vista has bugs in the APIs. In some cases it has fixed bugs which now cause problems with software applications.
2) The UAC is causing a number of problems
3) Where applications store their data has changed completely. Although this may not sound like a big thing, it often is because of bad coding historically.
4) Aero and new shell controls needs to be catered for. A lot of development tools are not doing this just yet so it has to be done by hand.
5) Certain areas cannot be written to. So anyone who has an auto-update has to change the way their software works because the program folders can not be changed in the same way they used to.
There was a 4 month period in the beginning of the year where I could not sell HP printers, scanners and multifunction to my clients, because HP neglected to bring out Vista drivers. During that time Canon and Kyocera had working drivers for all their printers (new and old), even before Vista was released. they had drivers available for the beta versions too. I was a long time supporter of HP products, but due to their recent performance (or lack thereof), I switched my client to Canon and Kyocera products. Why could they do it and HP not???
Because of the number of drivers HP had to convert, test, and sign. The whole driver system has changed completely so in a lot of cases, drivers had to be rewritten. This is not a simple task. I can assure you that guys like HP did not like the fact that they did not have drivers out there. At least they released drivers. Lexmark for example, did not create any drivers for discontinued products as far as I am aware of.