Windows 10 - now I have to pay for it

[#1]

In which case the computer won't boot without the disk holding Vista.  But that's not a problem because you can always use the bootrec tool to repair the dual-boot capability if it gets lost.  Or to make the Windows 7 disk bootable if you want to.

[#2]

So would the easiest option be to add the SSD correctly partitioned and then copy the Win 7 partition to it and disconnect the Vista drive then make it bootable using the Win 7 disk (if I do that would it be better to disconnect the exiting Win 7 drive just in case)

Once having done that I could set the boot order in bios to go to the SSD first and the Vista drive second just in case something goes adrift

[#3]

Yes, that should work fine, @Oldjim.  It's probably safer to disconnect the existing Windows 7 drive.

You should perhaps consider the relative merits of moving your existing Windows 7 to the SSD or starting again with a clean install.  The latter would take much longer but you would shed any dross accumulated over the years so the end result would most likely be better. 

[#4]

To be honest the thought of having to reinstall everything and setting everything up to work fills me with horror so I won't do it

Win 7 still works fine with no real slowdowns

[#5]

Yep, if it ain't broke.....