Modern Vespa

The clutch cable on my '79 P200e broke. I replace it but now the spring tension is very light. What else could have gone wrong?

If it pulled properly before it broke, it's probably the simplest explanation. First just take more slack out of it, most likely at the pinch bolt. You'll want to put light tension against the clutch arm to compress the light return spring, then get the pinch bolt as snug to clutch arm as you can while pulling back on the cable. Make sure your adjuster is screwed most of the way in so you can screw it out to take up any slack when the new cable stretches as it settles in.

I did do that. The clutch are is very easy to move without the cable attached. Last time it had a lot more tension. This time, practically none.

Does the clutch actually work?
My guess; you haven't really adjusted it enough

If you're right, it sounds like time to pop off the cover & have a look. Should be light resistance part way against the return spring, then heavy resistance as you depress the clutch springs. Maybe something inside let go and helped your cable to snap if it was already starting to let go.

If you previously removed the cover to have a look, the brass plunger may have gotten misplaced and left out. That would explain what you are seeing nicely and should be easy to fix if you can find the little devil.

when I adjust my clutch cable now on the pinch bolt i wedge a stick between case and clutch arm and then it hold its Tight while I tighten the pinchbolt. Makes the whole process a breeze.

I've used a zip lock to tighten the tension between the clutch arm and then installed the pinch bolt. Cut the zip lock and then use the adjuster to adjust tension, and you're good to go.