Hello Frederic
As VTP packets are being exchanged, you should see revision numbers increasing, and you should at some point see VLAN information included as additional sections of the VLAN Trunking Protocol section in Wireshark like so:
You can take a look at this example of an exchange of VTP packets for more details:
Now I don’t know why your capture resulting in what you described. How are you capturing, using SPAN or RSPAN? In order to correctly capture control packets like VTP, you must use the encapsulation replicate keywords at the end of the destination port command. You can find out more information at this Cisco documentation:
Let us know how you get on with your troubleshooting, and if you have further questions, you know where to find us!
I hope this has been helpful!
Laz