Io's Tvashtar volcanic plume displayed an irregular order of magnitude increase in brightness during the 2007 New Horizons (NH) flyby as the solar phase angle of Io relative to NH increased. High-resolution NH/LORRI images captured the Tvashtar plume's evolution throughout the flyby. We investigate potential causes for the brightness surge by examining whether changes in Tvashtar's volcanic vent properties could have explained the observed brightness increase. Using the captured NH images, a robust method is developed to constrain some of Tvashtar's vent properties. This method involves a sensitivity study of free parameters at Tvashtar's vent using axisymmetric Direct Simulation Monte Carlo (DSMC) simulations incorporating both gas and grain radiation modeling. The analysis examines how variations in these parameters (such as vent stagnation temperature, area, mass flow rate, and grain mass loading) influence the plume canopy's height, width, and overall shape. The sensitivity analysis identifies the vent stagnation temperature and area as the parameters most influential on the plume's canopy characteristics. Given the visibility of Tvashtar's plume canopy throughout the NH flyby, canopy spatial coordinates are extracted from each LORRI image. Subsequently, a Levenberg-Marquardt optimization algorithm is employed to fit DSMC simulation plume canopies, parameterized over a two-dimensional space of stagnation temperature and area, to each extracted canopy. This process yields the optimal pair of vent stagnation temperature and area that best models the observed plume canopy for each case. From the fitting process, we hypothesize an asymmetric source region at Tvashtar, consistent with previous research findings. However, the fitting process also determines that the observed increase in brightness cannot be entirely attributed to changes in vent conditions during the flyby, as there appears to be no correlation between these changes and the surge in brightness. The most plausible explanation for the brightness surge lies in the optical scattering properties of the plume particulates.