You can use Thin Reclamation with the following API:
uint vxfs_ts_reclaim(char *mountpoint, uint64_t offset, uint64_t length, int32_t volindex, uint64_t unit_size, uint64_t *bytes_reclaimed, uint32_t flag)
This is a non-reentrant API. This API cannot be called when an instance of the fsadm command or a reorg of the file system is running.
VxFS may align the offset and length internally for correctness.