I tested on fw 3.8.8.2 bt and on the latest 3.8.9.4 bt, on booth is the same. Actually I changed MTU to 1492 on vigor but not sure if that matters in bridge mode ,maybe that is the problem? Should I try non bt fw on it?
Try changing MTU to 1508 on BT Firmware first. then if exists, enable baby jumbo frames option, Alas trial and error I am afraid 
Then if that fails, replace BTÂ firmware with standard stock firmware (non-vectoring firmware) for testing and repeat as before
————-
Edit: Also, unsure what router is connecting to DrayTeK, but worth checking you are using best MTU for router:
from cmd prompt (windows) ping google.com -f -l 1472
The -f marks packets that should not be fragmented in the ping. -l 1472 sets the size of the packet.
If you get successful replies, then your current MTU is fine for your connection. If you receive error messages like in the above image, then your packets are getting fragmented.
Keep trying to ping until you get 4 successful replies. Keep decreasing the MTU by 10, so if 1472 fails, try 1462.
You shouldn’t go below 1400.
When you find a value that is successful, start to increase that value by 1, so if 1462 is successful, for example, try again with 1463 etc until you get errors again.
3) When you find a successful value, you can then set a new MTU limit with this value.
You will need to add 28 back on to the value for IP/ICMP overheads, so if 1462 is successful, then 1490 is your MTU limit.
Then change routers MTU to 1490 
In a completely sane world, madness is the only freedom (J.G.Ballard).
You need to login in order to vote