vconfig, invalid arguments and Favouritism

So I'm messing with vconfig;  really I'm letting the system do most of it, but it backs onto vconfig.

Explain to me why I can't vconfig a new vlan to an interface:

# service network stop ; service network start
Shutting down interface eth2.2401:  Removed VLAN -:eth2.2401:-
                                                   [  OK  ]
Shutting down interface eth2.2402:  Removed VLAN -:eth2.2402:-
                                                   [  OK  ]
Shutting down interface eth2.2403:  Removed VLAN -:eth2.2403:-
                                                   [  OK  ]
Shutting down interface eth2.2404:  Removed VLAN -:eth2.2404:-
                                                   [  OK  ]
Shutting down interface eth0:                      [  OK  ]
Shutting down interface eth1:                      [  OK  ]
Shutting down interface eth2:                      [  OK  ]
Shutting down loopback interface:                  [  OK  ]
Disabling IPv4 packet forwarding:  net.ipv4.ip_forward = 0
                                                   [  OK  ]
Bringing up loopback interface:                    [  OK  ]
Bringing up interface eth0:                        [  OK  ]
Bringing up interface eth1:
Determining IP information for eth1... done.
                                                   [  OK  ]
Bringing up interface eth2:                        [  OK  ]
Bringing up interface eth0.2401:                   [  OK  ]
Bringing up interface eth0.2403:  ERROR: trying to add VLAN #2403 to IF -:eth0:-  error: Invalid argument
ERROR: could not add vlan 2403 as eth0.2403 on dev eth0
                                                           [FAILED]
Bringing up interface eth2.2401:  Added VLAN with VID == 2401 to IF -:eth2:-
                                                   [  OK  ]
Bringing up interface eth2.2402:  Added VLAN with VID == 2402 to IF -:eth2:-
                                                   [  OK  ]
Bringing up interface eth2.2403:  Added VLAN with VID == 2403 to IF -:eth2:-
                                                   [  OK  ]
Bringing up interface eth2.2404:  Added VLAN with VID == 2404 to IF -:eth2:-
                                                   [  OK  ]
Bringing up interface br1:                         [  OK  ]
Bringing up interface br3:                         [  OK  ]

That's not the weirdest part:

# ifconfig | grep HW
br1       Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:00
eth0      Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:00
eth0.2401 Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:00
eth1      Link encap:Ethernet  HWaddr 00:0C:29:78:8F:9E
eth2      Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:14
eth2.2401 Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:14
eth2.2402 Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:14
eth2.2403 Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:14
eth2.2404 Link encap:Ethernet  HWaddr 00:0C:29:5C:6D:14
tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
tun1      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
tun2      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
tun3      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
# vconfig add eth0 2404
Added VLAN with VID == 2404 to IF -:eth0:-
# vconfig rem eth0.2404
Removed VLAN -:eth0.2404:-
# vconfig add eth0 2403
ERROR: trying to add VLAN #2403 to IF -:eth0:-  error: Invalid argument

For some reason, that exact vLAN is the one I can't apply to that interface.  The preceding one goes fine;  so does the one after.  That one?  No go.

Riddle me that.  And, once again. sorry if the format absolutely sucks.  I just can't figure out how to make this editor not chew up my blockquotes.