I'm no networking expert but have been doing some reading and think I've worked out the answer and would like confirmation if possible.

I'm trying to setup a guest wifi network with the above hardware.
Am I correct in thinking that because the 2820 is older and only supports one subnet without the VLAN tag feature then I would have to run two ethernet cables to the AP800? one of them going to the LAN B port after configuring the VLAN options in the 2820?

if is important to use second subnet i am not sure with Lan B, never use it
but if you use it normal just enable guest at the 2820 then at the ap800 setting it with this way
http://www.draytek.com/index.php?option=com_k2&view=item&id=2777&Itemid=293&lang=en
give same pass, username, channel, key for normal wifi and then enable guest at the ap and so same process at "Setting Wireless LAN on Vigor2830n"
That is wireless but i think one cable to Lan should be ok as well or try wired at AP mode

What s the current firmware at the 2820?

it is on the latest firmware, however I do actually have 4 AP800's and they are all set as AP's not repeaters. i Dont think I need it to be a repeater as its wired.