Every day, Perry Blatz knows where his son Bill will be — stuffing and labeling envelopes at the UPMC Vocational Training Center in the South Side, a facility for adults with intellectual disabilities.  “It's safe. He enjoys going there, he talks about going to work,” said Mr. Blatz, a retired history professor.  When he was younger, Bill had tried to work in other jobs, such as in a grocery store, but that didn't work out. Because of his developmental disability, Bill has a hard time staying on task. He has been at the Vocation Center full time since 2008.