Take a look at the parts diagrams for your headset/speedo set up so you can familiarize yourself with it:
http://www.scooterhelp.com/serial/VBC.VLB.VNC.parts.book.html
There are a bunch of pages showing that system, here's one:
http://www.scooterhelp.com/manuals/VLB.VNC.VBC.parts.manual/page36.jpg
(I say don't take off the '44' if you can avoid it, if the drive gear is in there and working, leave it alone:
http://www.scooterhelp.com/manuals/VLB.VNC.VBC.parts.manual/page24.jpg )
And yes take the head light off then..
There is a screw underneath the headset that holds the speedo in. Undo that so you can take the speedo out enough to loosen the retaining ring that attaches the cable housing to the speedo. It might help to feed the speedo cable housing up from the fork a bit to get some slack so you can get the speedo out of the headset.
Did the entire cable break? The housing and the inner? or is it just the inner you need to replace?
If you have to run a new housing go get a piece of wipper snipper plastic line (or something similar to that). Use a length about twice as long as the housing and feed it through the old housing. Tie the bottom end of the wipper snipper to your foot or something, then pull the old housing out off that line up through the headset. This way you will have that plastic line as a guide to run the new housing through the fork.
Its a bit tricky to get a new speedo cable housing through the little opening in the side of the fork sometimes, you might want to grease it up and also try to grab it through the little fork opening with a pair of needle nose pliers if it doesn't just pop through easily. You can also try tying the headset end of the guide wire to something bigger then the speedo retainer and tugging on the lower end of the guide wire.
Running the speedo inner once the housing is in is simple. Slick it right up with some lithium grease or something similar as you feed it in there.