For the direct fiber coupling of small optical measurement cells, we developed a new compact vacuum feedthrough for glass fibers and other similarly shaped objects that are compact and that offer the possibility of adjusting the fiber in longitudinal and in circular direction. The feedthrough assembly avoids compression or torsion on the fiber and thus protects, e.g., highly frangible fiber materials. In the following, we will present a brief simulation of the tightness requirements for low-pressure and low-concentration water vapor measurements and we will explain an integrated concept for a displaceable and self-adjustable, compression-free, compact, ultra-high vacuum, resealable feedthrough with good strain relief. The feedthrough has been successfully tested in a laboratory test facility and in several extractive airborne tunable diode laser absorption spectroscopy hygrometers. The leakage rate of the feedthrough presented here was tested via a helium leak searcher and was quantified further in an 8-week vacuum measurement campaign. The leakage rate is determined to be 0.41 ± 0.04 × 10(-9) hPa l/s, which--to our knowledge--is the first time a leakage rate for such a feedthrough has been quantified.