New
#11
Macrium's Reflect and a new one called ToDo are both seeing free system imaging programs for that purpose.
If there's nothing on the 7 installation quite yet an even faster method of seeing 7 use up the remaining drive space since the Vista primary has been reduced would be simply nuking the 7 primary rather then moving it for a new where a fresh install of 7 would go on that much faster.
Some here report a clean install of 7 in as little as 12 minutes while the average is right around 20. Now compare that to the time required to move the 7 primary forward and expand it afterwards or the 39-45 minutes typical of an XP or Vista fresh install. New primary to replace first for 7 only no need to move or resize simply created in the total empty space.
As far as imaging the drive you would want to do something like that once you have both Vista and 7 the way you want to see them and in top working order first. Then you would have the optimum backup image created at that time. A general data backup for anything you want to prevent from being lost would be the first step in case you run into any big problem where either Windows won't run.