If you use the correct shaders in the setup, you should be able to use anything as a skybox, it’s just a texture.
I have heard many people talk about Vue but never found a link to their site, could you provide one?
In Unity 5, the new PBS skybox shaders are located under Skyboxes in the shader dropdown in materials.
As far as having a perfectly aligned sun, design your skybox WITHOUT a sun, then tell Unity to add it in procedurally using a directional light, so no matter where it is, the sun graphic moves with it.
You can use the 6 sided, cubemap and procedural shaders for this, but personally I would recommend using the cubemap one.
I believe you can also use the dynamic sun on all of these, but I cannot remember how, ill take a look and get back to you.
UPDATE: add the skybox without a sun to a cubemap, then add that to a material.
Open up Window/Lighting and add the material in Environment Lighting/Skybox and the directional light to be the sun under Sun.
Change the Ambient Source to Skybox so your skybox material will effect reflectant materials in the PBS shaders.