I definitely recommend using Acronis if you are to be doing any partition work with Vista (PartitionMagic uses a different way of writing the partitions than Vista does and will have trouble reading partitions created by vista and vise versa).
Vista can be tricky to move around from HDD to HDD or even just simple changes the partition or others it works with. After imaging Vista to the 300gb HDD, you may have to rebuild the boot information. Vista no longer uses a boot.ini file. It uses a better, yet, new and harder to alter boot system. None-the-less, if you encounter problems, it's almost guaranteed that you can fix it. It just might take a while, and it just might take some guidance.
Things to keep in mind if you run into problems: Make sure the Vista partition is the primary partition on the primary HDD. Also, the executables, 'BCDEdit.exe', 'Bootmgr.exe', and 'bootcfg (with possible /rebuild parameter)' may come in handy.
Good luck. Let us know if you have any trouble, and... Although the above may sound scary, take a few deep breaths. You may not have to do any of that.