Modern Vespa

Just reel off all of the issues to them and get a bit of experienced honesty.
I always feel like they're saying " it's not my fault that the expensive repairs I've just done haven't worked again " and with a " we could always try this "approach.
Painful

Make sure you get in there somewhere "The Vespa Gurus at Modern Vespa say..."

• It shouldn't take more than 30 minutes
• Any mechanic should be able to fix that
• Spending 4 hours diagnosing the issue shouldn't be paid time
• All of the above
  

No mechanic has a crystal ball, specially when it is old scooters, it could finally come down to a damaged or rusty wire, and that takes time to find, mechanical is normally easier than electrical to find, lots of help here is a good start to help narrow down the problem, before talking to mechanic, if you can go in there and talk the same language as the mechanic, there is less chance of being ripped off.

"The most important customer is the one that pay's the most, the 2nd most important is the one that annoy's the most."
Quote from a former service manager when I was a shop supervisor.

I had a worn key + lock on my 2013 GTS replaced with a new set and had the new keys coded to the ECU.

Then I had a problem with the antenna ring not reading the keys.
I had it replaced (at great cost) but insisted on getting the old part returned.

A vespa guru friend told me afterwards that he would remove the ring, use switch cleaner to clean the connections, let them dry for 30 min before reconnecting.

Often it is just corrosion on the connections that is the problem - not the part itself.