The interaction between free electrons and photons is strongly constrained in free space, but it can be dramatically modified when the surrounding electromagnetic vacuum is structured, driven, or synthetically engineered. In this talk, I will present a few recent studies from my group that together develop this viewpoint. First, I will discuss a universal upper bound on the quantum vacuum interaction strength between free electrons and photons for both 3D and 2D materials, which identifies the material, geometric, and energy conditions required to approach maximal vacuum coupling and reveals distinct optimal regimes for slow and fast electrons. Next, I will show how a driven nonlinear environment promotes free-electron–photon coupling into a parametric quantum regime, enabling quantum amplification, detuned gain/loss channels, nonclassical state generation, and the possibility of a deterministic gain-only dielectric laser accelerator. Finally, I will discuss synthetic gain in electron-beam spectroscopy, where the effective photonic response can be reshaped in post-processing to uncover weak excitations in electron energy-loss and cathodoluminescence spectra. Together, these results suggest that the vacuum seen by a free electron can be shaped at the levels of mode structure, dynamical driving, and spectral response, opening new opportunities for strong coupling, quantum light generation, and enhanced spectroscopy.