Wide-angle high-precision differential phase-delay astrometry with VLBI at 7mm wavelength has been successfully demonstrated with state-of-the-art observations and analysis of a complete sample of 13 radio sources (Quasars and BL Lacs). With 20 mm wavelength observations separated by 10 years, the proper motions of the cores of the radio sources have been determined with unprecedented accuracy and with simultaneous 20/7 mm wavelength observations the core-shift effect has been studied using three independent methods. The limits of the technique with the present instrumentation will be discussed.