Preheat oven to 350°F (177°C). Line a 9-inch square baking pan with foil or parchment paper and then spray with nonstick spray.
Spread the oats and pecans on a large rimmed baking sheet. Bake in the preheated oven for 6 to 8 minutes, shaking halfway through, until golden and fragrant. Transfer to a large bowl; add the chopped cranberries.
Bring the milk to a boil in a small saucepan set over medium heat. Stir in the multi-grain cereal and remove from the heat; let stand for 2 minutes. Add the nut or seed butter, maple syrup, cinnamon, and salt. Cook and stir the mixture over low heat for 6 to 7 minutes or until thickened; remove from the heat and stir in vanilla extract.
Immediately add the cereal mixture to the oats mixture, mixing with a spatula until coated.
Transfer the mixture to the prepared pan and flatten it with the back of a measuring cup.
Place a large piece of parchment paper or wax paper atop the bar mixture and use it to spread, flatten, and very firmly compact the bars evenly in the pan. Cool completely. Refrigerate for at least 1 hour or until firm.
Using the liner, lift the mixture from the pan and transfer it to a cutting board. Peel off the foil or parchment and cut into 16 bars.