When this pseudowire has no outer MPLS tunnel (vcOnly case), this object holds the ifIndex of the actual physical or logical outbound interface the VC label is transmitted on directly; it's zero whenever an outer tunnel exists instead. It's the object to check first when a vcOnly pseudowire isn't forwarding traffic, since it tells you exactly which local interface's health is relevant to this specific VC. If this ifIndex points to an interface that's administratively down, that alone fully explains why a vcOnly pseudowire shows outbound failures, with no need to look further into label or tunnel signaling.