I made it work but there is still a problem with setVocabularyStrings of INVocabulary. It looks like contacts added with that method won't be passed to INSendPaymentIntentHandling.resolvePayee (payment app). Except from this issue (wating for a fix from Apple right now...) it looks ok. The steps to do are:
1) Create an app extension to manage Siri calls in your project
2) Add Siri Kit capability to the App (fron XCode 8)
3) Add Siri app extension to your app (XCode Wizard)
4) Add the required attributes to the info.plist of the extension for Siri support (see Apple documentation on NSExtension)
5) Add NSSiriUsageDescription in your app info.plist (see Apple online documentation)
6) Add NSExtensionPrincipalClass (see apple documentation) to your app extension info.plist file. It must point to your Intent handling class implementation iside the extension
7) Implement the required extension's protocol
You can find a few examples online.
After that your app should be able to integrate Siri. Another story is to add new items in the Vocabulary. I'll tell you what to do with that after Apple will answer me.
UPDATE
There is another glitch. Even if you ask Siri to make a payment in a foreign currency (Euro for example), Siri will initially understand you are requiring a foreign currency payment... but then she will ask you on screen confirmation for a payment in US dollars, no matter which currency you initially dictated. That is, she will understand you are payng with another currency, but in the end she will revert to USD when it is time to complete the payment.