Apachez, the QOS-MAPPING is FROM:TO, which means egress-priority 0 will be mapped to 2, egress-priority 2 will be mapped to 3. In my case i need only 0:3 or 0:5 on vif-s or vif-c because priority 0 is used by default for iperf application. Hope this helps for your understanding...

Viacheslav, YES it works, THANK YOU so much, now it's possible to set the priority on vif-c and vif-s. This is a great step forward to have this opportunity in my lab.
Now i have two questions:
-Is there a possibility to make the setting permanent ?
-And do you think, someone in the community is willing to implement a command/parameter to configure the priority on vif-c and vif-s also CLI ?. As i wrote I would pay for this implementation so everyone in the community can use this feature in VyOS.

The priorisation for data traffic on our DSLAM access network is based on 802.1q because this are mainly Layer2 devices. The network between the DSLAMs prioritize also the data traffic based on 802.1q header. At the moment we need Layer2 ethernet tester which allow us to set the COS bit (0-7) on vif and vif-c+vif-s to measure the QoS through the network. The reason for my request was, to ask some VyOS community member, if this would be possible with VyOS + iperf like we do it today with the Layer2 ethernet tester.

Yes, we want to configure the QoS priority "three bits" of the 802.1p field. In our lab the packet forwarding is based on 802.1q priority.
https://en.wikipedia.org/wiki/IEEE_802.1Q

Beside Juniper, other router vendor starting with there specific cli shell. But if the JunOS approach is much easier for maintenance on the long term, take this way.