Hey there!
There are obviously both advantages and disadvantages to having a part time job during VCE. It's important to consider a number of factors including how many shifts you will be getting a week, the flexibility of the job and the number of other extracurriculars you are dedicating your time to.
If you are not doing too much outside of study, a job can provide a mental break from year 12 as well as build skills you can take with you after high school. At the end of the day, if you are working less than 10 hours a week this shouldn't have a major impact on your study. Especially if you manage your time well and the workplace is flexible (happy to give you time off before exams to study, easy to swap shifts with people prior to SACs, etc). Personally, I worked about three shifts a fortnight during VCE and found that this was a good balance for me, but I know other people who decided to wait until after year 12 to get a job.
Really, it just depends on your own situation and finding something that works best for you. However, I do want to stress that having a job does not necessarily mean that your grades will drop. It's impossible to study 24/7 anyway and having a job can provide you with a routine and structure that actually decreases procrastination. There's also no harm in getting a job in yr 11 but realising that it doesn't work well for you and deciding to drop it in yr 12.