RetailEdge will run fine on Window Vista. We run RetailEdge here on Vista machines. Most of the time there aren't any issues. However, we really want our customer's out of box experience to be as good as possible, and this is why we are currently still recommending that users use machines that have XP when they can. If there are problems we typically see two types of problems with Vista.
1. Security issues. Vista is designed to be more secure than XP. However, that typically leads to issues for people who setup user accounts on the machines that have less than administrative rights. For instance, RetailEdge has an automatic update feature that allows you to download updates to the program from our website. However if you download this update with a user that is not the administrator, then the program does not have the permissions to do what it is supposed to do when it tries to replace the program with the newer version. In some cases, RetailEdge might have permission to delete the program, but not to copy the new file into the program files group. Another problem related to this is during setup, if the program is not installed with a user that has administrator rights, then some of the folders that RetailEdge creates during this process do not have read/write access and the program won't run properly.
2. Driver Issues. We have seen some devices that have drivers that either are not Vista compatible or the driver for Vista is not the greatest. For instance, I was just working with a cash drawer that was supposed to work with Vista but did not. I had to send the end user back to the manufacturer and she was on with them for hours until they resolved the problem.