OK, I'm beginning to lose the will...
I know the 2762 is an entry-level device, and it does not have any of the more advanced router options of its more expensive stablemates. However, should I be able to use the EE Brightbox 2 to act as the VDSL modem and let the DrayTek handle the LAN? I know there is not a modem mode setting on the BB2, so I can't use WAN2 on the DrayTek, but if I were to set IPs to static and disable DHCP and NAT on the BB2, how can I set the DrayTek to route the Internet via the LAN port on the BB2?

I suppose it would be like setting the DrayTek as an access point, but using the EE router to handle the DSL Internet connection.

The value for UDP settings are listed in the Defence setup section:

Enabling the UDP Flood Defense option may affect UDP services, like NAT-T VPN, DNS, and VoIP service.
When you have UDP servers running, please increase the UDP threshold according to the Internet speed, or add the server IP into the White List. The suggested threshold value are as follows:
20M Bandwidth: 2,000 (packets/sec)
60M Bandwidth: 5,000 (packets/sec)
100M Bandwidth: 8,000 (packets/sec)
300M Bandwidth: 25,000 (packets/sec)

My 2762 is dropping connections, too, but I believe it's to do with some interference on the line - I'm getting disconnections from late afternoon until early hours in the morning, with some being at regular times and intervals. When I used the same 2762 on the ADSL line, it held onto it very well. So, is it more susceptible to noise on the VDSL line? We're getting a Plusnet router soon so I'll test the connection with it and report back.

When the SNR on our line was automatically increased by the DSLAM to 12db, we didn't experience any disconnections. At 6db, it's constant, but in the evenings only, as I described above.

This just keeps getting better. As a result of a frustrating phone call to the EE support and threats to go to another ISP, I received a call from the loyalty team. They told me my 5 year old Brightbox 2 was not optimal for my connection type and broadband package, so they sent me a new Smart Hub. I connected this up to my PC (isolated from the rest of the network) I just wanted to see if it offered a modem-only mode, which is doesn't seem to. In fact, the user interface and even the advanced settings area seems to be very limited in what you can see, in terms of connection settings etc. It is hugely simplified, but shows that EE don't really want their customers playing around with things.
Before this arrived, I had already followed up a suggestion on the EE community forum, and got hold of an Openreach (Huawei Echolife HG612 3B) modem. I was advised to literally plug it in and connect its LAN1 port to the DrayTek LAN4 (WAN) port. I tried this, and set the WAN2 settings:

WAN2 General
Physical Mode: Ethernet
Active Mode: Always on
VLAN Tag insertion: Disable

WAN2 Internet Settings
Type: PPPoE
Username & Password
WAN Connection Detection: PPP Detect
MTU: 1492
Everything else on that page is default.

I am monitoring the connection on the HG612 and on the DrayTek, but so far, it has been stable.

Ideally, as this solution cannot be upgraded further, due to the legacy nature of these Huawei boxes, I would really like to be able to use the either the Brightbox 2 or the new Smart Hub as the VDSL modem, but I appreciate that I would need to consider the DrayTek as an access point (additional router). What is the best way to do that? How do I connect the DrayTek in such a way that devices connected to it will be able to access the Internet through the EE box?

I contacted DrayTek support and asked for some options to fix the problem. The solution was to employ the EE router to handle the DSL and then connect the Vigor 2762 WAN2 (LAN4) port, to the EE LAN port (any).

Here are the steps I followed:
1) Set the EE Router to 192.168.10.1 and Enable DHCP
2) Connect the EE router to LAN Port 4 on the DrayTek
3) Login to the Draytek Router
4) Go to WAN > General Setup > Enable WAN2 > OK
5) Go to WAN > Internet Access > WAN2 Details Page > Static or Dynamic IP > Enable
6) Select Obtain an IP address automatically and click OK
7) Click OK to reboot the router

WAN2 should now obtain an IP address from the EE Device and route through to the internet.

This has been working now without any disconnections. I know there may be some NAT issues, but for my sort of use, I don't think that this will be a problem.

Thanks to Shailesh on DrayTek support for all the help and advice.