Really, no VIL threshold for heavy rainfall can be established. It will vary significantly depending on what type of environment you are in. Heavy tropical rains from shallow convection will yield much lower VIL values than heavy rains in higher latitudes resulting from deeper convection. Remember, VIL is a calculation of a storm's water content in the vertical.
There are three kinds of VIL. Grid-based VIL, cell-based VIL, and digital VIL. I'm not overly familiar with digital VIL as our office prefers legacy VIL. Grid-based VIL is often viewed in the plan view and consists of bins that are 4 km by 4 km.
As for the highest VIL I've seen, grid-based VIL is not calculated above 70 kg m^-2. I've seen 70 kg m^-2 numerous times. Cell-based VIL has no limit that I'm aware of and I've seen it as high as 115 kg m^-2 before in the plains during late spring. Cell-based VIL better accounts for tilted updrafts and thus will often be higher than grid-based VIL values in storms which occur in highly sheared environments but may be equal to or even possibly lower than grid-based VIL in storms occurring in low-shear environments.
