Cut circles out of the 4 slices of bread that are approximately the same size as the veggie burger patties. Butter one side of each bread circle and assemble 2 grilled cheese sandwiches with the cheddar slices. Heat these in a pan over medium heat until golden brown on the outside.
Assemble with one grilled cheese as your bottom "bun", spread some vegan mayonnaise on the inside, then add the veggie burger patty, tomato slices, pickle slices, and lettuce. Mix 2 parts ketchup and 1 part sriracha for the spicy ketchup, spread that on the inside of the top grilled cheese "bun" and place on top.