I transferred from RMIT to Melbourne Uni, so I am happy to give you some info:
They look at your past 12 months of study. This means that if you were apply to study Bachelor of Arts in semester 2, they would look at both your VCE scores and your uni grades. You apply through VTAC as per usual.
The fact that you have enrolled in another tertiary institution makes you ineligible for Access Melbourne from now on. The minimum ATAR for Bachelor of Arts is 85.00, which means that if you had an 84.95 ATAR and 100% at La Trobe Uni, you still wouldn't be accepted into Bachelor of Arts at Melbourne Uni for the mid-year intake. If, however, you do have an 85.00 ATAR or higher, then your ATAR will count for 50% of the weighting and your university GPA will count for 50% as well.
As a general rule, the entry requirements for start of year intakes are much lower than mid-year intakes. In addition to this, if you complete a full year at La Trobe, then Melbourne Uni will only look at your uni grades, which means that the 85.00 minimum ATAR will no longer apply to you (the only thing they'll check is to ensure that you met the 25 English prerequisite). Whilst there is no specific score that will guarantee you a place at Melbourne Uni, a 75-80% average at La Trobe will give you a good chance of transferring into Bachelor of Arts for the start of year intake.