Hey there,
It seems there is currently a bug in FRR that causes issues when trying to enable BGP EVPN in a different VRF than the default one.
There is however a workaround for this, if you first add the `router bgp 1 vrf stuff` statement to the config and then `router bgp 1` it should actually work, the FRR bug has to do with the order in which they get configured.
If you first define `router bgp 1` and then `router bgp 1 vrf stuff` (which is what VyOS does) then you get a `Please unconfigure EVPN in VRF (null)` error.
The FRR developers are working on a solution (there is no issue on GitHub yet as far as I know) however this might take a while to get released.