while this is related to bug 893576, the custom property should be visible in other hooks, like start vm

How I can call from hook script custom properties of specific vnic?

For example vm custom properties I can assign from script with os.environ['name_of_custom_properties']

1) Perform on rhevm: 
engine-config -s CustomDeviceProperties="{
    type=interface;
    prop={
        speed=^([0-9]{1,5})$;
        port_mirroring=^(True|False)$;
        bandwidth=^([0-9]{1,5})$}}"
restart engine

2) For each vnic can choose custom properties speed, port mirroring and bandwith.

3) Also can receive for each vnic custom properties from hook script via os.environ['key']

Verified on is9

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2014-0038.html