I am using the IP routing 2nd subnet facility on my Vigor 2820n to utilise the 8 public IP addresses my ISP (Zen) has given me- x.x.x.168 to x.x.x.175
Draytek's instructions say I should put the first available address (so x.x.x.169 ) in the "2nd IP address" box.
This address is given to the 2820n & becomes the gateway for the devices using the other public IP addresses.

However in Zen's Knowledge base "Vigor-2800-Series-Routed-IP-setup" it says the "2nd IP address" should be "the Router's external IP address as supplied in your technical details e-mail"
Now Zen always allocate the LAST usable address in the subnet ( x.x.x.174 in my case) to the router's WAN, not the first.

So,
do I put x.x.x.174 as the "2nd IP address" which will confuse the 2820n if it thinks this is the first address of the range,
or,
do I force the 2820n's WAN to x.x.x.169 and confuse Zen,
or,
do I put x.x.x.169 as the "2nd IP address" and so use up 2 different public IP addresses on the 2820n (169 & 174)?

Any thoughts welcome.

I would say with IP range 168 to 175

Your network is 168
Broadcast address is 175
Usable host addresses 169 to 174
Therefore put 169 in as the address for IP routing in the Vigor.

I would expect your sub-net mask to be 255.255.255.248

That certainly works but seems rather wasteful, giving the 2820n two of my public IP addresses.

Subnet is, as you say, 255.25.255.248, hence network is 168, broadcast is 175.
Of the 6 usable addresses, this technique gives the first & last to the 2820n, leaving me just the 4 in the middle.

You always lose two addresses.

for example if your local LAN was 192.169.1.x

You couldn't use either 192.168.1.0 or 192.168.1.255

And certainly having small public sub-net is pretty wasteful, but that's the way it works...

Losing 2 out of 8 is unfortunate.
Losing 4 out of 8 seems positively careless.

168 = Network
169 = Router
170 = Available
171 = Available
172 = Available
173 = Available
174 = Available
175 = Broadcast

So you have 5 usable, and it's probably reasonable to expect the router to take one.