As seen on slack:

The Broadcom NICs are 2x 200Gbit/s, they are initialized proper and the bnxt_en driver loaded as well. I just want to upgrade the firmware of the card itself. Broadcom offers a tool named niccli (previously known as bnxtnvm) but it’s not compatible with VyOS. Or at least the sliff-dkms package is not working without dkms.

So root cause seems to be if dkms is added to VyOS then one could upload the firmware update tool (niccli) and sliff-dkms package in order to update the firmware of the NIC through VyOS.

Without the above a workaround might be to get another bootmedia and boot devices on liveboot Ubuntu or such but this might be an issue if you have multiple devices at multiple locations - be able to perform the firmware update from within VyOS (even if it involves manually uploading the binaries to the VyOS boxes) would save alot of time.