In a large pot, melt the butter over medium/high heat
Add onion, carrots, and celery, - cook the vegetables in the butter for about 10 minutes, or until they are tender
Add the cornstarch, and whisk in to incorporate it into the butter
Gradually add the milk, 1 cup at a time - whisking with the butter mixture
Increase heat, and bring to a boil (Watch closely. Once the milk starts to boil, it can overflow from the pot. If this starts to happen, reduce the heat so it settles down).
Add the bouillon cube
Stir constantly for about 1 minute or until the milk mixture is thick and smooth and the bouillon dissolves
Lower heat to a simmer, add vinegar, seasonings, and crab meat - Stir in, and simmer for about 5 minutes - stirring occasionally