For data that might have been on your current hard drive, it's certainly very common to add a second brand new hard drive to install Win7 onto, while leaving your current hard drive also installed.
That way you can easily just access the data on the old drive which will get probably a drive letter of D as seen by Win7 (which will install itself onto what it sees as C, on the new hard drive). Or, if you want, you can migrate that data off of the old drive and onto the new C partition (Win7) or even onto another partition(s) you create on the new drive. Or, just leave it on the old drive, but you might want to place it into more appropriately named folders if it's in a place now that seems inappropriate for the future.
If you want to end up with a dual-boot system (with both Vista and Win7 available from a boot manager menu), that's one installation method for Win7. If you want to end up with just a single Win7 system (with the data from the old drive just available as data), that's another installation method.
But assuming you just want to move to Win7 and forget about Vista, you should change your BIOS setting before you run the Win7 install, to set your new hard drive as "hard disk #1". This will cause the Win7 installer to create both the (a) 100MB "system reserved" partition, which will be marked as "active" on that drive, as well as (b) the Win7 system partition itself, on that new drive.
The existing Vista system on your old drive should be ignored by the Win7 installer, because you've made the new drive "hard disk #1" in the BIOS. You're not doing an "upgrade", you're doing a clean from-scratch new install to an empty new drive. It shouldn't be necessary to disconnect the old drive first, once you change "hard disk #1" in the BIOS to point to the new drive. Both drives can be present during the Win7 install. But if you are cautious, skeptical, or dubious, you can of course disconnect your old drive during the Win7 install and reconnect it after you're up and running on Win7. My own feeling is that it should not be necessary to do this.
You can set the partition size for the Win7 partition during the install, leaving the remainder of the new drive for yet another new data partition and drive letter. This will be in addition to the old hard drive, which if it's currently only one partition will simply be yet another drive letter in your new Win7 environment.
Once you get Win7 up and running from the new drive, you'll probably want to clean house on that old drive, deleting the Vista-specific folder and files, while leaving your data. Personally, I'd recommend moving the data from the \Users folders (e.g. Documents, pictures, etc.) to ordinary external folders on that drive, or into comparable \Users folders (on the new Win7 C) of Win7. Up to you.
But for sure, you'll have no more need for the Vista contents on the old drive. You should just be careful you're not deleting your real data (which might be buried inside of some \Windows folders/sub-folder on that old drive) when you clean house.
