The version of fastnetmon in current(vyos1.4) is 1.2, the latest opensource version available is 1.1.3 (can be downloaded from: https://github.com/pavel-odintsov/fastnetmon/releases)
As i've found out the version of fastnetmon we are using is precompiled by the developers of that package and only a amd64 package is provided of the package.

This makes a missing dependency whan building on arm64 platforms, as the dependency in vyos-1x is marked as all platforms.
This is also an issue for anyone that wants to build vyos themself as they cant get hold of that package from any other place than our apt repo
We then will be dependent on a package from a 3rd party we do not have control over witch is not good.

I will with this suggest to roll back the version of fastnetmon to version 1.1.3 that is freely available online or getting hold of arm64 packages for this package.
As a temporary measure we also need to mark the dependency in vyos-1x as amd64 only.