The HCl vapor grenade would be the cheapest and easiest to "Ghetto-fab" of these munitions. It would to absolutely HORRIBLE things to unprotected humans and animals-- but it would also permanently destroy engines that were operating within the resulting fume cloud.
Said cloud would be colorless and invisible, but not odorless. (and you would DEFINITELY know if you stumbled into one!) Rapid production of pure HCl gas can be accomlished with metal catalysts and heat in the presence of sufficiently dense chloride salts. Suitable catalyst can be obtained from old catalytic converters. (which is why I say the device is easily ghetto-fabbed)
Hydrochloric acid is actually a gas that has been dissolved into the water, in much the same way that carbolic acid is carbon dioxide dissolved in water. PURE gas would be highly hygroscopic, and would be pulled into any mucous membrane exposed to it, causing instant free radical deterioration of the tissue. (In other words, concentrated pure gas would destroy lungs and eyeballs in seconds.) When sucked into an operating vehicle, the gas would combine with the trace quantities of water vapor produced by the combustion of the engine's fuel. producing liquid HCL solution inside the engine's calendars and exhaust system, which would continue to corrode and damage the engine even after the vehicle drove out of the cloud. It would shred valves and valvestems, rapidly rust holes in the exhaust system, and pit/corrode the calendar walls in short order.
The materials needed to construct such a device would be very easy to source. Most of the chemicals found in the pool aisle at walmart would be suitable chloride sources.
Again, Geneva convetion violation, in great big neon letters. The Geneva convention ban on chemical weapons came about after WW1, when molecular chlorine gas was deployed.
Concentrated HCl vapor would be right up there in line with molecular chlorine.