Having a second HDD is a nice convenience since you can then install Win7 to it, browse to recover your files from Vista, then clean reinstall Vista to the first HD to set up a BIOS dual boot that leaves the HD's independent to come and go as you please - whereas a Windows-managed Dual boot interlocks them. You can also back up image and data to the other HD in case you lose one.
But if you can't get Vista started you can also achieve last-resort file rescue by simply reinstalling Win7 to its partition and using it to rescue files - unless you also have files on Win7 you don't want to lose.
You could then boot Vista DVD to delete all other partitions, create a new one for Vista plus a middle data partiton if you'd like to store your data there in it's own "vault" linked to User folder in both OS's. This way reinstalls/reimaging due to failure of either OS in the future wouldn't touch your data.
But meanwhile the Partiton Wizard drive map may fill in the gaps as to why Vista won't start, or help us get Win7 started so that you can rescue your data, then possibly reinstall Vista.
There is also a free Paragon Rescue disk that can copy over files as well as rescue the partitions themselves since they are intact. Or you can use your Win7 DVD now to copy over files: Copy & Paste - in Windows Recovery Console