simply put when I run anything on the behaviour tree it wont move on from the current task. in this case vendor response will send out a message(the talk to vendor event) and end but on the behaviour tree it will never move on from that task. What am I doing wrong here?
Make sure to have the Finish Execute node at the end of your task 
I have it at the end of my task with success bool checked yes
Could you share your blueprint code? If you have branches for example in your task, you should place a Finish Execute node at the end of every execution chain. You can also check if it actually gets triggered by using breakpoints.
I tried to straight up replace the custom event with a simple print string of hello in the task but I had the same issue. the first screenshot is of the custome vent I am calling in the BTT and the second one is of the actual task itself.
So did it print? 
Additionally, could you check if the other task under your sequence composite gets triggered as well?
yes it did print, i have swapped them around and both can get triggered its just when one of them gets triggered it wont move onto the next one which is the issue.
Why post a screen shot of BTT_DruidOneTalksToVendorOne? Isn’t that the task that never gets a chance to run cause BTT_VendorResponse won’t finish? What is in BTT_VendorResponse, you need a Finish Execute node in that task too.
One of those casts have to be failing. You have one BTT that cast to BP_NPC_adventurer_one and another BTT casting to BP_NPC_VendorOne. Both are using the same pawn, the controlled pawn, as the object.
