I am planning to use my Vigor 2860 ac behind a Community Fibre FTTP modem,
in place of the Linksys Velop router they provide. I read that I need to set the Draytek
DHCPv6 client to Prefix Delegation mode.

Regarding your router, you should only need to set up the WAN port for DHCP for IPv4 and DHCP with Prefix Delegation (PD) for IPv6. No PPoE or anything. If you need something more specific, it's probably worth posting in r/CommunityFibre where I am sure lots of people might benefit from seeing the result, and I'll try my best to answer too.

However, there is no specific DHCPv6-PD setting in my firmware, nor in the v4.5 version of the user manual.
As there are telnet commands to request identity association IDs for Prefix Delegation AND Non-temporary Address, am I right
in thinking that DHCPv6 client will do whatever is needed without my doing anything?

Any further tips gratefully received.

Henry

I have used the 2862 with ADSL and IPv6 on WAN1. I guess with an external modem you are using WAN2?

I have had a quick look at the 2860 manual and think that if you are able to set connection type to PPP it will use DHCPv6 and request a prefix. The more complex PD settings relate to the LAN side when you have another router downstream.