Indeed, it is difficult to determine Vclay reliably, using the GR
log, if the main clay mineral is Kaolinite.
You may want to try Vclay estimation using the density and neutron porosity logs.
Another method is to use a multi-mineral model approach.
If you have a Sigma log, it may help as the Sigma for Kaolinite (around 14 c.u.) is significantly different from those for quartz, calcite and dolomite.