This paper presents a review of recent advances in development of sub-THz miniaturized vacuum electron devices (μVEDs). We focus on O-type μVEDs, which operation principle is based on interaction of a linear electron beam with a slow electromagnetic wave. Traveling-wave-tubes (TWT) are of the most interest as high-gain and wide-bandwidth amplifiers. The most attention is paid to sub-THz TWTs with folded-waveguide slow-wave structures (SWS), TWTs with grating-type SWS and sheet electron beam, and TWTs with planar microstrip SWSs. Backward-wave oscillators (BWO) are used as low-power radiation sources with broadband frequency tuning. BWOs provide power of several mW at frequencies up to 1 THz. Extended-interaction klystron amplifiers and extended-interaction oscillators offer high power at sub-THz frequency range but at the cost of narrow bandwidth. Best performances achieved by various research teams are summarized.