You should be able to use the blocked_range as youcreated above together with auto_partitioner.
You could also use the method range(size_t grainsize)of concurrent_vector with auto_partitioner; just specify grainsize of 1 for it. This method will do for you what you did manually, i.e. construct a blocked_range over iteration space [ v.begin(), v(end) ) with a given grainsize. The value of 1 is the default grainsize value for blocked_range; we should have made it the default value in the concurrent_vector's range method as well.
Both ways are essentially the same.