Poplar wood is commonly used for many purposes due to its easy machinability, low density, uniform light colour, and relatively low cost. Here, vacuum thermal treatment is proposed for upgrading veneers in the manufacturing of plywood panels with resulting reduced hygroscopicity, improved durability, and dimensional stability. Thirty-eight batch processes with different treatment conditions (temperature ranging from 150 to 240 °C, time from 0.5 to 22.5 h and pressure from 100 to 1000 mbar) were performed to characterize the influence of process parameters on the product properties. Samples were characterized considering their appearance (colour) and their physical (mass loss and equilibrium moisture content), mechanical (bending strength), and chemical (investigated with near infrared spectroscopy (NIR)) properties. The darkening of poplar veneers and the reduction of mechanical strength were observed with increased treatment time and intensity. Mass loss closely correlated with colour change, resulting from chemical changes in wood components. Principal component analysis (PCA) and partial least squares (PLS) were used for evaluation of near infrared spectral data. Both were correlated with several technical properties, and thus NIR allowed the simultaneous prediction of several of these properties. Both colour change and NIR could be used to optimize the thermal treatment of poplar veneers at the industrial scale and for real-time statistical process control.