A confusion between dB and dBVrms
When you start to use non RMS values to calculate decibel ratios (or relative values) then you get muddled up. For instance why would you want to compare the peak amplitude of a sinewave (Vs) with the RMS value of 1 V RMS? Maybe that is your confusion... When you say 


  Vx = Vs*sin(wt) where Vs is the amplitude


Vs is the peak sine amplitude and not the RMS amplitude - are you aware of that?


  When we plot FFT power spectrum which one is relevant dB or dBVrms?
  What is the difference in use?


A power spectrum is usually relative to some RMS reference so it's not all that useful to express it as plain dB unless one is interested in the relative amplitudes of parts of the spectrum.