I know this is a very old thread but I happened on it wanting to find the answer (VBA inverse log of x) and Google seems quite confident this thread answers the question. Others probably will find themselves here so I am taking the liberty of providing the proper conclusion for those yet to come this way . . .

The earlier answer neglects the important fact that the VBA logarithmic functions yield Natural (Naperian) logs. To calculate Common Logs and Inverse Common Logs use the following:

CommonLogx = Log(x) / Log(10)

InverseCommonLogX = Exp(x * Log(10))