Vocab is a great thing to address early. Compiling lists is a helpful starting point. Plenty of resources out there for L.A. persuasive devices or words to describe the way views and values are communicated through Section A texts. Keep adding to these constantly; any new words that you find interesting, especially ones used in other essays. You won't see results right away, but it will make a big difference in the end.
It's perfectly fine to be unsure of your abilities at this point though. In fact, throughout most of Year 12 you'll be reassessing where you're at, so it's not like you have to decide on three weaknesses now and never think about this again. Rereading practice essays is probably the best place to start. Unfortunately if your teacher has been less than helpful with the feedback then you'll be on your own for this first part. Maybe compare your approach to higher scoring essays; what are they doing differently, and how can you incorporate these techniques to suit your style?
Tutoring is by no means a necessity to do well. For the most part, it'll depend on the quality of your teacher next year.
I'd be very careful seeing another teacher. If you're on good terms with one of them then by all means go for it, but you don't want to risk annoying your current teacher (who will presumably be marking your Year 11 exam?) by making him feel inferior. English teachers are sensitive sometimes...
The Submissions Board here can help you out, but attempting to identify these concern yourself should defintely be the priority over these holidays.