In VLC if a video itself does not contain those black borders (when not in fullscreen mode), you have to create some first where subtitles will be displayed. You can do it in preferences:
Tools > Preferences (Ctrl+P) > (Show settings - All) > Video > Filters > Cropadd (Video scaling filter) - Pixels to padd to bottom [200] << for example this will add 200 pixels of black border to the bottom of the video.
Now you need to turn it on:
... > Video > Filters: in Video filter module section tick Video scaling filter, so you get [croppadd] in the text bar under the list of available Video filter modules. Be careful because there are two Video scaling filters in the list. The other one puts [swscale] into the text bar and you do not want this one.
Save settings and restart VLC. Later do not forget to turn it off for other videos where you do not need to add the black bar.
Now you should see the video with added black bar on the bottom. If you switch to fullscreen mode you can enjoy subtitles under the picture. There are also hotkeys available to move subtitles up and down on the screen. Please check these hotkeys also in preferences.
Recently I have opened a ticket for this, requesting to make also 2 hotkeys for PADDING to get subtitles under the picture user-friendly way: Ticket #6172 - Custom Aspect Ratio and Padding "on the fly". But I also understand that this is not one of the highest priorities for developers.