This is the same problem as reported in T4297. By definition it is not possible to change speed and duplex settings at SR-IOV virtual functions driven by ixgbevf driver. I think the solution is the same as well, that is to add 'ixgbevf' into _drivers_without_speed_duplex_flow in /usr/lib/python3/dist-packages/vyos/ethtool.py. It fixed the problem for me with Intel x520 NICs. Let me know if you need any additional information. Thanks!