The K800i is compatible with Vista, the guy in the shop was blatantly rubbish.
I'm guessing he meant that the software may not have the correct drivers for Vista and you may need to download the relevant drivers!
I'd say any phone company that's making new phones would be pretty stupid to NOT make them Vista compatible!
Anyway, here's a link for the K800i and making it Vista compatible;
mobile9 Forum > Sony K800 / Vista / Capi Worker Module?