CsYbSe2, a recently identified quantum spin liquid (QSL) candidate, exhibits strong crystal electric field (CEF) excitations. Here, we identify phonon and CEF modes with Raman spectroscopy and observe strong CEF-phonon coupling resulting in a vibronic bound state. Complex, mesoscale interplay between phonon modes and CEF modes is observed in real space. Additionally, an unexpected resonant Raman excitation condition is satisfied, yielding up to third-order combination modes, with a total of 17 modes identified in the spectra. This study paves the way to coherent control of possible QSL ground states with optically accessible CEF-phonon manifolds and mesoscale engineering of CEF-phonon interactions.