Modern Vespa

This peculiar phenomenon happens on every petrol filling.
My -12 model of 300GTS did not have it but this years model has.
Interesting thing, and more interesting that it has not choked engine once.
Should drill a small hole to the cap, or leave it very loose?

As I understand it, a '12 GTS sold in Europe did not have an evap control system to reduce/avoid venting gasoline fumes into the air while a '24 GTS does. The different process for tank venting is probably what causes the different you are describing. On the other hand, you acknowledge that you see no sign that the newer system adversely affects performance. What would you be trying to fix by intentionally defeating the evap system?

Yes, at least hopefully the tank has not being trying to srink, got wrinkled and started to leak, so all is nicely as it should? Why even check the oil, if motor has not yet being seized once?

If you have a new GTS under warranty and you truly suspect the tank venting system is not functioning properly, why don't you consult your dealer for piece of mind?

True thing, silly me of at all think that here would be anybody knowing Vespas tanks. Good tip to leave this.
Btw: I drilled hole to tank cap. No hissing...