Hi Fatbananas, my answer is "I don't know".
This is what I intend to do - and this is very different from probably what will happen in reality as my child will probably not "play ball" when the time comes.
I am going to buy the practice papers which are closest to the real paper - for us this is the GL assessment practice papers. I am also going to work through the GL assessment book "VR explained" which explains in detail how to do each GL assessment question type. GL assessment sets the Kent paper, so this way I am doing the closest possible preparation for the real thing. This book also suggests some games and things which help in the preparation - but they are suggestions specifically linked to the question types rather than blind pieces of advice like "expand your vocabulary".
Then I'm going to go through all the practice papers myself, long before I ever put them anywhere near my child, and make notes on how to solve each question and which question type they are from the the "VR explained" book.
I think I will then be a little closer to answering your question. I think the answer is that focused practice and appropriate teaching for the weaknesses will make a huge difference. I am not sure how much vocab will matter yet.
By the time I've got round to doing all this ( a long while off
) if we are at the stage where the practice papers are too hard ( as we're only year 4 now) I'm going to take a good look at the Bond VR books for the year 4 age group and see if they fit well with the GL assessment practice papers. If they don't I'm not going to use them. Instead I'll have to think of my own way of building up gradually to tackling each question type.
This site is great, but of course everyone is sitting different papers from you, so you might read advice on here which is, say, right for Buckinghamshire, but not right for whatever school / county you are going for.