As is appropriate on ng's when the OP finally gets the solution to a problem etc...
Here's how I solved the following... Setting up WDS between an Airport Extreme base station and a Vigor
2600g wireless router.
(above for easier search string hits).
Ensure you have the CORRECT MAC addresses before attempting to set up ANY WDS!
This turned out to be the most basic problem at the root of my trials and errors. The admin pages for the Vigor router config interface does not clearly announce the MAC address for the WIRELESS point of the device. This is to be found at the top of the MAIN "Wireless LAN Setup" screen, under the heading "Wireless Lan Information". Even the guy at Draytek Premium support was unsure where to find this, when first asked. As you are easily led to 2 other different MAC addresses on pages preceeding this, it's easy to assume that one of them is the WLAN MAC address!
Once I tried this MAC address in the Airport admin utility, I was up and running within about a minute. The only other thing you need to focus on between the AEBS and the Vigor, is to ensure that they are both set to the same channel. Disable all security settings for setup purposes, you can configure them later, once you have the WDS working. Also, don't name your AEBS network, the same as that broadcast by the Vigor (it's easy to assume that this is what you should do). Once the WDS has been setup, you use the AIBS's airport network... that sent out by the vigor remains in your drop down list of available networks, but is not required (unless of course you roam outside of the range of the AEBS, but are still within that of the Vigor, which you can then use).
The connection between the AEBS and the Vigor happens beyond your awareness... they converse via the MAC addresses (or DHCP) and all you do is work off your newly named AEBS network.
There are many options on the vigor config pages that are indicitive of having some relevance in setting up a WDS, and it's tempting to tamper with them when you're running out of ideas... DON'T... the vigor tech guy stipulated that nothing need be changed other than entering the MAC address of the peer (AEBS) in the "Bridge" section of the setup pane, after enabling WDS in "Bridge" mode. That's it! do nothing else with the Vigor. Everything else gets done in Airport utility, and that's supported by Apple and the support pages of the Apple site.
If you've been using your vigor as a wireless router before now, you'll likely already have IP's distributed around your network via DHCP and NAT... this remains the same. Your AEBS (or express base station for that matter) gets its own IP this way. In the Airport Utility, after setting your BS up to "participate in a WDS network" and entering the vigor's WIRELESS LAN MAC address into the WDS pane and the AEBS as a "relay", you then go to Internet Connection and, at the bottom, set "Connection Sharing" to "off (Bridge Mode)".
I reiterate my first point... Make sure you have the WIRELESS MAC address of your router.
The rest you should be able to work out, after all, you're a MacUser :)