For years, Oakland became a national punchline for rampant vehicle break-ins. Visitors were warned not to leave anything visible inside their cars. Some residents went even further, leaving trunks open to show thieves there was nothing worth stealing. Now, those break-ins are falling sharply. According to KTVU, vehicle break-ins in Oakland are down 37 percent compared to a year ago. For residents, commuters, and tourists, that sounds like unambi…