If you fully stripped a Vespa and a Lambretta I reckon the Lambretta would have nearly triple the parts (nuts and bolts included). The Lambretta engine certainly contains more moving parts and bits than the Vespa.
With this in mind I would suggest Lambrettas are more difficult to put together and work on (and that's the fun of Lambrettas

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The Lambretta's are, in general, easier to tune, or at least there are more options.
I agree the Lambretta's handle better than Vespas. I suspect Lambrettas have a longer wheelbase and it feels as though you sit higher on a Vespa ?
I did thousands of miles on Vespas, first a Rally 200 then a PX, in the late 70's and 80's and cant recall one breakdown ? The Rally handled better than the PX and was a tad quicker.........wish I still had it !!