Preheat the oven to 350F and line a baking sheet with parchment paper.
In a small bowl, combine the juice of 1 lime and Cajun seasoning. Spread the mixture on all sides of the salmon pieces.
In a large cast-iron skillet, heat 1 tablespoon of the vegetable oil over medium-high heat. Add the sliced bell pepper and onion and cook, stirring constantly, for 5 to 7 minutes, until crisp-tender. Transfer the vegetables to a plate and tent with aluminum foil to keep warm.
Heat the remaining 2 tablespoons of vegetable oil in the skillet. Add the salmon pieces and cook for 3 to 4 minutes until the bottoms blacken, then flip and cook the other side for 3 to 4 minutes until blackened. Transfer the salmon to a plate and give it a rough chop into smaller pieces.
While the veggies and salmon cook, put the grape tomatoes on the prepared baking sheet; drizzle with olive oil, and sprinkle with the salt. Roast until the tomatoes start to burst and turn black on a few sides, 5 to 10 minutes.
To assemble the tacos, start with a later of pepper and onion on each tortilla, top with some salmon, charred tomatoes, and avocado slices, and drizzle with the dressing. Serve with lime wedges.