Cook shallots and oil in a small saucepan over medium-high heat, stirring occasionally, until shallots are golden, 8-10 minutes. Using a slotted spoon, transfer shallots to a paper towel-lined plate; season with salt. Transfer oil to a small bowl and let cool. Combine collard greens, radishes, and vinegar in a large bowl; season with salt and pepper. Drizzle with shallot oil and toss to coat. Top with crispy shallots.