initscripts don't support "$VLAN_ID". They actually support "$VID",
which NetworkManager doesn't.

"$VLAN_ID" was introduced by commit 10b32be3 ('ifcfg-rh: various VLAN
cleanups'). It has a comment about "backward compatibility" for the case
where the reader would ignore "$VLAN_ID" if "$DEVICE"'s name contains
a suffix that is parsable as VLAN ID.

That is wrong. If a new feature gets introduce (like NetworkManager
supporting "$VLAN_ID"), then there is no way that an older version of the
tool -- which doesn't know the new feature yet (initscripts) -- supports it.
This is not what backward compatibility means. Backward compatibility
means that if a user has an old ifcfg-file without "$VLAN_ID", then we
continue parsing it as before.

Consider, when a user (or NetworkManager) writes a configuration

  DEVICE=vlan9
  PHYSDEV=eth0
  VLAN_ID=10

then it makes no sense to ignore VLAN_ID=10 and use "9" instead.
Otherwise the user (or NetworkManager) should not have written the
file this way.

Also, NetworkManager profiles support "connection.interface-name=vlan9"
together with "vlan.id=10". Such a configuration is valid and must be
expressible in ifcfg-rh format. The ifcfg-rh writer code did not somehow
restrict the setting of "$VLAN_ID" to account for this odd behavior. Whenever
NetworkManager in the past wrote VLAN_ID variable to file, it really meant
it.

https://bugzilla.redhat.com/show_bug.cgi?id=1907960

NetworkManager/NetworkManager!794