Returning a populated VStack from a SwiftUI function

You were close to the solution, actually. Indeed, you can use some View as return type this way:

func buildResponseText2() -> some View {
    Group {
        if userData.success {
            VStack {
                Text("Well Done")
                Button("Play Again", action: {self.userData.reset()})
            }
        } else {
            Text("Unlucky")
        }
    }
}

Use type erasing AnyView as return type, as below

func buildResponseText2() -> AnyView {
    if (userData.success) {
        return AnyView(VStack {
            Text("Well Done")
            Button("Play Again", action: {self.userData.reset()})
        })
    }
    return AnyView(VStack {
        Text("Unlucky")
    })
}

Extract layout and Preserve the original type:

You should implement a custom view with a custom view builder so you can return different types. Also, you may want to remove the VStack from the return value, so you can decide to use a different layout later without any need to change the content.

So implement this:

struct StackContent: View {
    let success: Bool

    @ViewBuilder var body: some View {
        switch success {
        case true:
            Text("Well Done")
            Button("Play Again") { print("clicked") }
        case false:
            Text("Unlucky")
        }
    }
}

Then you can have this:

func buildResponseText2() -> some View {
    VStak { // <- you can change it to other layout containers
         StackContent(success: userData.success)
    }
}

_{Maybe you don't need the function at all anymore ;)}

Note that there is no need to write @ViewBuilder from Swift 5.3 (Xcode 12)