Sounds like you've gotten a lot of great ideas here already. I would add to keep in mind that when you put two different fabric types together, at least one of them tends to hate life. I would think that the scrim would tear at the poly webbing because of the weight below from the vinyl. Of...
Using a blue vinyl cyc or a white vinyl cyc is truly just a choice. Only difference really, is that the blue will tint your stage lighting a bit. Of course this depends on what colors your lighting designer is using.
Twin White is much more commonly used.