Some late-to-the-thread-again comments:
Bradypus - sounds like a great bivy set-up for walking in the cold and dry season you picked. Nice one!
Hobbledehoy - as dick bird says, you should expect rain. Although Spain currently experiencing a drought, it's highly likely to end in the Spring. Many stories in past years of folk having to improvise because of flooding.
If you are liking the idea of bivying: lots of cheap Tyvek bivy bags (or just a sheet) around. Wash before use to reduce the crackle ;-) . And/or no harm in taking a few grams of polycro as well. If you have time to order, money to spend and, are taking a walking pole, consider Bearpaw's
Lair. Mine with 'beak' and all pegs is under 300g. It's a truly excellent item.
IMO, if you are the weight conscious type (aiming for an 8kg pack), I'd ditch the whole bivying out idea: just go with the state of mind that nearly all of your fellow pilgrims will have - that you have to work with the accommodation that is available, and adapt accordingly. March was always a popular starting month for the VDLP, so you should find many more options than Bradypus is finding now.
Cheers, tom
PS When I did VDLP my pack was 2.8kg. That's before I put anything into it. After about 200km I finally posted home a bivy bag, a gas cooker and a boxed chromatic harmonica - don't ask.