When creating the new approval process you select the field used for the approval routing in step 3. Under "Create New Hierarchical Relationship Field" you add the label for your custom field (in this example, "VPAPP"). When you get to the 'Approval Steps' part of the set up you can then select the, "Automatically assign using the user field selected earlier." Works like a champ. As a bonus, once you have done this for one approval process, that field is available to select for future approval processes.
Hope this helps.
we ended up adding a lookup field in incident that looks at the user table. then building a process in the process builder to fill it in from the VP field of the user in the client field. this essentially copies the VP field from the user table into the incident allowing me to reference it in the approval process.