I've tried this on multiple Compex WP54 family devices, a Ubiquity PowerStation 2 and a LiteStation 2.

Starting from a simple configuration with the wireless correctly associated with an access point (the device having my problem is in Station mode). ath0 has an appropriate IP and can ping and be pinged by other devices.

However, when I create a VLAN interface on ath0, things start to go wrong (vconfig add ath0 5).  Approximately 40-60 seconds after the vlan interface is created, the wireless traffic from the device stops.  Running tcpdump on the device montioring the wireless shows incoming ARP requests for the device's IP address, but no responses.

Almost immediately after destroying the vlan interface (vconfig rem ath0.5), the network traffic resumes.

This is all on OpenWRT r10517.  (These devices all use Atheros wireless chipsets...)

Well.. based on looking at tcpdump on both the wireless AP and the PS2 (as a client), it appears that the PS2 is attempting to do an ARP request on the router's address.  The router appears to be attempting to send back the ARP response, but it doesn't appear in the tcpdump output running on the PS2.

Does anybody have any ideas on either what this actual problem is, or how I could potentially continue to attempt to debug this?

Wat version of OpenWRT you are using for your WP54 ?

Do u try for the Kamikaze 7.07 from Compex it self or u are using the OpenWRT one.
As what i know it fix some Vlan Bug. but not really sure wat is that .

U may try to ask from the Tech support for the file if u dont have.

As mentioned above.  r10517 on both.  And this has been failing as far back as r8309 on the Compex.

YOu may try on r7117 or V7.07 this 2 are more stable ( my personal view)