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Vellum is a little like Memory Foam - in time it will lie relatively flat once more. There are several ways to speed the process without wetting it - "cold steam" humidification and magnets works very quickly (be sure to use waxed paper between the magnets and the vellum, and between the vellum and the steel plate upon which it rests) as does high relative humidity and sandbagging (nothing more than a layer of common sand atop a sheet of buffered paper covering the leaf while it rests on a marble or formica tabletop), adding sand daily to increase the weight. The latter is slower but yields better results as the weight of the sand is more even than using magnets, and will "push down" all but the tightest, stiffest curns. In either case, be sure the "bow" faces upwards (high in the center).
Don't disparage it; it's a nice leaf. From CANTUS: A Database for Latin Ecclesiastical Chant:
http://cantusdatabase.org/id/004266
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