Vat photopolymerization (VPP) 3D printing is an additive manufacturing (AM) process in which liquid photopolymer is selectively cured using light-activated polymerization to create a desired 3D part. Since the curing reaction is exothermic, the process of printing will generate heat and affect the resin rheology, curing kinetics and printed part properties. It is important to know the temperature of the printing process, especially at the resin-part interface where the temperature distribution could indicate the anisotropic degree of conversionand residual stress among other temperature-related properties. However, it is difficult to measure the print interface in a desired non-contact approach due to the limited viewability and accessibility of the curing zone being sandwiched between solid build head and liquid resin. Existing work based on simulation has limited accuracy due to model assumptions and unknown/uncertain parameters. In this work, we demonstrate a framework of VPP-AM process temperature measurement methods that combine in-situ thermal monitoring and finite element-based model simulation. An in-situ temperature monitoring protocol is established to calibrate or obtain the thermal properties of VPP material system as well as measure the reachable surface temperature (e.g., resin chamber substrate, build head). These in-situ measurements are input to a physics-based model and simulation is conducted to estimate the desired temperature profile of the resin-part interface. To validate the developed thermal monitoring-informed simulation approach, we use a characterized bottom-up thermal imaging setup to monitor single-layer VPP printing, and our experiment result shows a good agreement between the simulation and direct measurement results.