Welcome back to our new Instagram fashion extravaganza, where we scour our current favorite app for the finest in Boston-area cyber snaps. This week’s Instagram fashion theme is vintage. You’ve seen these retro-loving individuals around town peering through cat glasses and strolling about in their finest 1950s thrift store garb. One of them, Sara Antoinette Stevens, is a 23-year-old Somerville resident and blogger. She has agreed to be our latest Insta-victim.