We present the Venus Emissivity Mapper (VEM) onboard NASAs Venus Emissivity, Radio science, InSAR, Topography, And Spectroscopy (VERITAS) and ESAs (EnVision) Venus orbiter missions. The VEM instrument (on EnVision called VenSpec-M), is a multispectral imager for mapping of the Venus surface and its lower atmosphere. This is realized by observation through narrow-band atmospheric windows present in the near-infrared spectral region around 1 μm. For the first time, VEM will provide a global Venus coverage of > 70% with a high signal-to-noise ratio on the order of 100 to detect thermal emissions like volcanic activity, surface rock composition, water abundance and cloud formation. Since VEM for VERITAS and VenSpec-M for EnVision are being developed almost simultaneously, the instrument development approach can be made very efficient. By tailoring the science level, interface and environmental requirements of both missions to a joint requirements baseline, a single instrument design can be established. Focusing on the science requirement breakdown, this paper presents the key scientific requirements derived from VERITAS and EnVision and how they translate into verifiable technical instrument requirements. The VEM/VenSpec-M project is in its preliminary design phase. The instrument preliminary design review (PDR) is planned in 2025 for VERITAS and EnVision. Two flight models (FMs) are currently scheduled for delivery to the VERITAS S/C in 2028 and one FM to the EnVision S/C in 2029. First VEM/VenSpec-M data obtained from Venus orbit is expected after launch of the two missions currently scheduled in 2031.