Out of the two, I think vocabulary is easier to tackle.
I used Vocabulary.com to actively learn new words and their meaning. Although, the site itself is good. I would recommend to either write down the words and their short meaning in a notebook or an excel table. The words you write should either be the words you found useful, or words that you might have heard before, but not used them or didn't know their precise meaning.
Now the reason why I recommend Excel table is for it's sorting function. Having an alphabetically sorted list is a much nicer and easier to navigate way than it's alternative, simple notebook. There is also a possibility to search for the keywords, which I found amazing.
The other thing you should do is to simply read more.
The thing I would recommend is to have an audiobook playing while reading the book. My reasoning behind this is that when I'm faced with a difficult word I might just skim over it. Taking its meaning from the concept of the sentence.
However, if I hear the word as well as read it. The word itself sticks in my mind easier. Meaning I might use it more often, therefore extending my vocabulary by it.
Regarding the style, I think you can only read and write more.
I remember reading the first Jack Reacher book by Lee Child. I loved its short sentences shadowed by, what seemed to me as, calculated force. The writing fit the main character who's an ex-military cop, a gentle giant ready to burst in action.
In my opinion, the style you use will mimic the authors you read, and that's not the bad thing. I also think it's hard to blunder the style. If your characters are believable and story sound you won't have to worry about your style.
In the end, I would recommend a couple of books, which helped me write clearer and be more confident when writing.
- Stein on Writing by Sol Stein
- The Elements of Style by William Strunk Jr.
The first book focuses on the process of writing, storytelling techniques and writing for yourself as well as the reader.
The second book is the collection of rules to follow when styling the text. How to format dialogues, use active voice and basic punctuation.
