You will probably think that using advanced vocabulary makes you sound, well, advanced.
However, this isn’t the case.
People will think you have a higher level if you use the right words, in the right way, at the right time.
For example, it’s better to say: “Hello, how are you?” in the right situation and with good pronunciation rather than something like, “What’s up, bro?” in the wrong situation and without good pronunciation.
You might think that the first one is too simple. But simple is good. Use vocabulary and sentences that fit the situation. It doesn’t matter if it’s beginner or advanced English. If it’s right, then use it.
This is true for slang too. I added the word bro because a lot of English learners use this in the wrong situations.
A good rule for using slang is this: know exactly what it means and how and when it used before using it yourself.
Again, you don’t need to use every new word or phrase you learn. Some vocabulary is best to stay passive. This is definitely true for a lot of slang terms.
This doesn’t mean that you shouldn’t use slang. If you know how to use it, go for it! But be careful because slang is often sensitive and used for very specific situations.