dBVrms is dB in reference to 1 Vrms.
so if I have 1V amplitude sine wave signal (ie 1*cos(wt)),
then Vrms=1/sqrt(2)=0.707=-3dBVrms


lo-rf isolation is measured in straight dB (a unitless ratio) NOT dBVrms.
so if 1% of the LO voltage signal leaks to the RF port then one can say the LO to RF isolation is 40dB.