vconfig add command fails adding a VLAN on loopback interface with RHEL 5.9

Issue

We experience a problem creating a VLAN on the lookback interface with command vconfig, the error is shown in the following example:

# vconfig add lo 20
ERROR: trying to add VLAN #20 to IF -:lo:-  error: Invalid argument

The same command was working on previous Red Hat edition

Kernel Linux 2.6.18-308.1.1.el5 #1 SMP Fri Feb 17 16:51:01 EST 2012 x86_64 x86_64 x86_64 GNU/Linux

, but fails with

Linux  2.6.18-346.el5 #1 SMP Mon Nov 12 21:37:23 EST 2012 x86_64 x86_64 x86_64 GNU/Linux

We use a VLAN on lookback as a way to create IP packages that can be capture by our application.