The scottish white pudding , non meat version would be a good substitute for vegetarian haggis so long as it is prepared with vegetable fat/suet so no animal fat.
It's very similar in taste because of the spicing and oatmeal. Called mealy pudding in the north east.
They even sell it battered in the fish and chip shops and with chips its called a white pudding supper.
The tinned vegetarian haggis sounds revolting.