This study sought to characterise the chemical composition of electronic cigarette liquids (e-liquids) available on the Klang Valley market, with a focus on identifying toxic compounds and understanding the key factors that determine flavour categories. The work addresses growing public health concerns regarding the proliferation of unregulated vaping products. Two hundred e-liquid samples collected from the Klang Valley were analysed using Gas Chromatography-Mass Spectrometry (GC-MS) to identify and quantify their chemical constituents. Decision Tree Analysis (DTA) was then applied to investigate associations between specific toxic compounds and different flavour profiles. Nicotine was detected in 196 of 200 samples (98.0%), with a mean relative concentration of 48.79% (SD 20.69; range 4.99–92.67) based on GC–MS peak-area estimation. GC–MS profiling identified 459 compounds in total, including 11 chemicals of potential toxicological concern. Decision Tree Analysis identified ethyl vanillin as the most significant variable for flavour classification (χ² = 80.688, df = 3, p < 0.001), with concentrations above 0.07154% more commonly associated with coffee, dessert, nut and tea profiles. Concentrations below 0.07154% were primarily linked to sweet, fruity flavours, whereas higher levels were characteristic of richer, dessert-like profiles. These findings indicate that the Klang Valley e-liquid market is heavily dominated by nicotine-containing products and flavour additives, with ethyl vanillin being a particularly prevalent flavouring agent. Of particular concern is the widespread presence of ethyl vanillin, as flavour aldehydes may undergo thermal transformation during vaping and contribute to inhalation exposure to potentially harmful degradation products. This underscores an urgent need for evidence-based regulatory standards, including limits on specific flavour additives to mitigate potential inhalation health risks.