The development of light-emitting polymers for optoelectronic devices requires the fine tuning of photophysical properties in solution and in the solid state. Poly(N-vinylcarbazole) (PVK) is widely used, but its solid-state emission is limited by aggregation-caused quenching (ACQ). This study investigated the effects of chemical substitution with bromine (PVK-Br) and furanyl (PVK-Fur) groups, focusing on emission spectra, aggregation behavior, and photoluminescence quantum yield (PLQY). PVK, PVK-Br, and PVK-Fur were synthesized and characterized in THF solution, as films (pure and with PMMA), and as powdered material. Under UV excitation (285–320 nm), the emission spectra and PLQY were measured, and the effect of aggregation was evaluated by titration with water. In THF, PVK-Fur showed intense emission with a narrower band (smaller FWHM), indicating greater color purity and less environmental disorder. The presence of the furanyl group promoted greater electronic conjugation and delocalization, shifting the emission to the blue region (hypochromic shift) and narrowing the band gap. The addition of small amounts of water (0.1 eq) increased the PLQY from 12.2% to 13.99%, indicating aggregation-induced emission (AIE), attributed to the restriction of intramolecular movements and the suppression of non-radiative pathways. However, larger volumes of water (>5 eq) caused intense aggregation, π–π stacking, and a sharp drop in PLQY, evidencing ACQ. In the solid state, even with PMMA, PVK-Fur showed low emission due to the strong suppression of excitons by aggregation. Thus, PVK-Fur stands out as a promising candidate for fluorescent sensors in aqueous media or microenvironments, where controlled aggregation favors emission.