If hpv can lie dormant for up to 20 years, does the gardasil hpv vaccine prevent it from ever appearing?

No it won't: Hpv /gardasil prevents the infection of the body with two important high risk strands of the virus (out of 100 there are 10 high risk strands). You need to get the vaccination prior to exposure to the virus strand. Once it is in your body the vaccination does not prevent it from becoming active. A decreased immune system could activate it so try to live healthy!