Dig a pit and build a fire in it*, or use a firepit with a built-in grate. Let fire burn to medium-high and spread out logs as needed; if using your own grate, set it in place.
Put butter, sugar, and salt in a large, heavy dutch oven or enameled cast-iron pot. Scrape vanilla seeds into pot and add bean. Set pot on grate over fire or on stove over medium-high heat. Cook, stirring, until mixture turns golden brown and frothy and begins to smell like caramel, 3 to 6 minutes.
Add fruit and stir to coat. Cover pot and cook, stirring occasionally, until fruit has released some juice and is glossy and glazed but not mushy, 2 to 4 minutes.
*The chefs use a grate with legs (Stansport Camp Grill, from $22; stansport.com). A charcoal grill grate set on bricks also works.
Make ahead: Cut fruit the morning of your outing (toss peaches with 2 tbsp. lemon juice) and chill airtight in a cooler.