When a veggie burger falls apart it means that there is not enough structure. It is either overly wet or overly dry. The balance is what you need to achieve in order to get a steady burger. Ingredients that add moisture to a veggie burger, like beans, veggies and liquid of any kind, need to be balanced out by ingredients that absorb that liquid, like breadcrumbs, cornmeal and flour. That is the first thing I do when creating a veggie burger. Once I have that down, I add all my seasonings and spices. Keep in mind that some flavor will be found in liquid form so make sure to add those when balancing out the moisture content.