Adobe Reader 11 wont run properly (though in some cases you may be able to install it) on Vista (Adobe dropped it from support for some reason). Reader 10 is the highest you can update to. However, you can use the built in firefox PDF reader, which will receive updates and remain secure as long as Firefox support vista (a long time)
I have Adobe Reader X 10.1.7 simply from updating it on my own and my platform is Vista. I have no idea why Mozilla keeps telling me to update the Plug-in and then shoots me over to a Reader 10.1.4 download.
I do have Acrobat X which I bought when it came out, and maybe that has something to do with it. In any case, clicking on the Firefox "update" box for the Adobe plug-in is logically incorrect, because you should at least be sent to a download for Adobe Reader 10.1.7, not 10.1.4.