Hi @Nyklon ,
VTIs are virtual interfaces which do not receive/transmit packets - they are logical only, for providing a nexthop to which traffic can be routed.
Traffic that is routed via a VTI is intercepted by the kernel, encrypted, and transmitted via the real physical interface of the gateway. Likewise, decrypted traffic is received on the external, real physical, interface of the gateway and transmitted by the internal, real physical, interface of the gateway. Because of this interception, no packet ever actually reaches the RX/TX queues of the vpntX interfaces, so the OS won't report them in the output of "ifconfig".
Hope this clears things up a bit.