There is no set number of post's/helpd ppl to get invited to vip, overall i think its all about if the staff feel your trustworthy.
ofc the more activ you are the better the chance you have.
as an example, ive been here for 6 months ish, helpd ppl here and there, found over 100 items missing on esp for eft and i havent been invited, but others who joind around the same time has, so as i said there is no set thing you have to do to get invited other then put in some work to earn it