Vocabulary is something you can never have enough of. Even the same words over time develop new or additional meanings. In the TOEIC test, you can get answers wrong because your vocabulary isn't broad enough. It may not necessarily be a hard word or phrase either, it might be an alternate meaning you're not aware of. So the more vocabulary you know the better. A large vocabulary range is something you can use for life, it's knowledge not limited to just being useful to get a high score.
It might also be an expression or idiom that you don't know that will trip you up on the test. In business, there's many expressions using common vocabulary that have different meanings implied. If you're going to work for a company, you'll need to understand such expressions. For example, "cook your goose" has nothing to do with a goose or cooking. It's a phrase meaning you've done something stupid or bad that will get you into a lot of trouble with the boss or maybe even fired. You could sign up for Testden's idiom of the day where we have many idioms and slang used in business situations. There's English idiom, expression, slang resources that you can readily find online. Here's a link to one with business expressions. This is one for phrases and slang, set up like a dictionary by letter.
For vocabulary you can expect on the TOEIC test, there is no exact definitive list you can learn. People have gathered common words used and the list is in the thousands. You can go a bit nuts trying to memorize them all such as the list on this web page. Doing sample TOEIC tests, like our free mini ones, is a better approach. By doing sample tests, you can find out what areas of vocabulary and grammar you need to improve upon instead of trying to cram in everything. Knowing how each type of word is used, and being able to properly guess the meaning of one you may not know from the context it is used in are much better skills to have than memorizing lists of vocabulary.
But you should definitely widen your vocabulary range and get a feel for what to expect on a TOEIC test. Online you can find lots of free material to help you with your vocabulary from ESL/EFL websites as well as ones specifically for TOEIC. Searching with TOEIC vocabulary will bring up lots of results. An example is this site.
No matter how good your vocabulary is, it won't get you too far if your grammar skills aren't sufficient. Many questions on the TOEIC test are designed to test a given aspect of grammar. Proficient grammar is a critical skill if you're going to succeed in business at an English company where you have to write e-mails, reports and give presentations. We have a practical and simple grammar guide you can use. The areas covered are all the main ones covered in the TOEIC test: verb tense, subject-verb agreement, word families, count and non-count nouns, comparative and superlative, adverbs of frequency, word choice, prepositions, unnecessary words, pronouns, conjunctions, and conditionals.