It seems I don't have much space. Further I can't delete vision as we use it for demo purpose.We cannot help you here as you need to add more space if you want to create a new instance on the same server.
1) how much space will be required to create new database?Are you installing a new database or cloning an existing one?
2) If I've space upto 450 GB and I re-install OS can I've 2 instances (Vision & Prod) then?No.
3) Can I delete some unusable tablespaces from Vision database to make some space? (sorry if it sound a foolish question - I'm newbie :P )Yes, if it is possible and you need to resize the existing tablespaces.