Personally, would encourage you not use File Vault unless you just have absolutely top secret data on your machine. If you have never used it before, I'd suggest you spend some time reading about some of the issues with it.
If you just must have it, am guessing that the issue is one of not enough contiguos free space. Try re-booting first, see if that may possibly auto delete the cache files which could be stored toward the end of the drive. After that, you might try using iDefrag or cloning your drive to an external and cloning it back with SuperDuper or CarbonCopyCloner.