where to use accoustical sealant on bath/shower ?

Hi I am new here so forgive me if I am asking this question of the wrong section of the forum..I am
installing vapour barrier in my bathtub alove prior to having the cement board installed.
My question is really two fold. 1. i have been told to use accoustical sealant to seal
the vapour barrier to the framing, where do I absolutely have to use this, studs sill, header? and 2. do I have to install vapour barrier below the tub sill. My drywall contractor
says no. I have seen posts on other sites where people have installed vp below the tub
sill. Help now I am really confused.
If this is the wrong section of the forum for this question please direct me to the correct one.