Let's say we have a separate text file with the subtitles in our language (soft subs). If we play those subs, we end up having something like this:
A couple of days ago, I found a simple way on VLC to cover the hard subs with a black rectangle containing the soft subs. It may not be the most elegant solution out there, but it worked...so, I'll share it.
First, on the top window of VLC, go to the menu "TOOLS" and then to "preferences" (you can use Control P if you are in a hurry.) By default, VLC will show you the simple settings. Instead, mark "ALL":
You will get something like this:
Now, on the right, you will see different options, colors, and values. We only need one: Background opacity.
By default, it will be set to 0 (meaning totally transparent.) The higher the value, the most visible our black rectangle will be.
I set it to 255, which is the highest value. Of course, you can change the color of the rectangle and of the font, but I found that a black rectangle with white letters works for me: