If you are speaking in a language that is not your first language or mother tongue, you may sometimes feel insecure that your vocabulary is not large enough. The truth is that there is no need to fret and worry.
The best speakers and the brightest speakers in the world, who may have a vocabulary consisting of hundreds of thousands or even millions of words, still typically use just a couple thousand of the smallest and most common words to get their point across when they are speaking. They understand that reading and hearing are very different and no matter how smart or eloquent your audience is, people hear differently than they may read.
It is harder to follow big words and long, complex sentences when you are hearing someone speak. So if you have a small vocabulary, you need to realize that this can be a huge advantage in this regard. It can make it easier for your audience to understand you and harder for them to get lost or tune out as long as you have done a good job isolating interesting, useful and important points for your audience.
So do not let a small vocabulary take off chunks of your confidence- it is actually a good thing in the world of speaking and can bring the ball in your park for you to hit.