Well actually there are two components of the Information Technology Certificate II Vet course, that of Certificate two in which for completing components\modules you either receive competent or not yet competent. While the HSC is different being made up of 50% assessment mark (which at least at my school comes from the trial which is worth 100% as we have had no assessments that go towards are HSC just competencies). Thus meaning your final HSC mark is made up of 50% assessment mark (based off my trial) and 50% of your HSC exam mark.
As for what your mates have been saying it really isn't true. As Certificate II isn't something to do with BOS\HSC and you can still complete IT without achieving Cert II. So 'doing good' in either the HSC or trial will only help your overall HSC mark and not your competencies on all topics, that you have to achieve through completing the various tasks and showing the teacher you are competent.
I hope that clears it up, good luck! And if anyone feels I am wrong in some aspect feel free to correct me in my tired state.