As a long time Acronis user and one of its biggest fans (even after I stopped using Windows) I can tell you that trying to shrink or enlarge Linux partitions with Aconis is almost bound to fail. Unless that is you happen to be using the Linux server version of Acronis which I doubt because it costs several hundred pounds. Despite their claims to the contrary Acronis does not work properly with Linux. It used to but now it doesn't. That change occurred when the decision was made to change to a 256 byte inode size in the ext3 file system.
I wouldn't do what you propose to do this time either. I would do it this way. Use Acronis(or windows) to shrink the Vista partition then use gparted to expand the Linux partition. Be warned though expanding partitions with gparted is extraordinarily slow. It really depends how much you have customised your Linux install, but if you haven't done much with it it is probably quicker to delete the partition completely, create a new one and reinstall!! No, seriously it is when you are used to the speed Acronis works!
Of course make sure that you have all data backed up anyway, and bear in mind that if you create any new partitions over and above the ones you have then you will have to edit /etc/fstab before you can boot properly again as the partition numbering could change.
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