The design and performance of a vapor generator for calibration and testing of trace chemical sensors are described. The device utilizes piezoelectric nozzles to dispense and vaporize precisely known amounts of analytic solutions onto a hot ceramic surface, where the generated vapors are mixed with air before exiting the device. By varying solution concentrations, microdroplet dispensing rates, air flow, and the number of active nozzles, vapor concentration profiles can be delivered on-demand across six orders of magnitude (12 to 25M parts per 1015). Known levels of impurities can also be added to challenge tested detectors. Microdroplets are monitored by microscope with strobed illumination, and the reproducibility of diameters (relative standard deviation 1 %) was optimized by varying piezoelectric waveform parameters. Complete vaporization of the microdroplets occurs only within the transition boiling regime of the solvent; for isobutanol, this occurs between 130 C and 140 C. We present ion mobility spectrometry (IMS) data of trace explosive vapors generated by the device, which exhibit linearity within the range of the IMS detector. Current efforts are directed towards minimizing wall effects and verifying the dynamic range of the device.