What's seen in VP preview depends upon the aspect ratio of the source, which can be changed via various effects. Use MATCH MONITOR for the sequence to see a good representation of what the export will look like.
Different export resolutions will produce differing results. Auto-Match is a suggestion based on source files, but you can choose. Usually the best quality will be obtained by closely matching export to the highest resolution of the source files.
Don't use VLC here, but it should have view settings that may eliminate the borders, or perhaps the window size can be dragged smaller.
If parameters vary on a number of previously exported files, they likely will appear as they were exported when reloaded, so there could be a mix of A/R on the final export. Experiment.
https://support.google.com/youtube/answer/1722171?hl=en
If you have allowed the project to fully cache (completed green line above the thumbnails) then it should respond well. That said, video editing being resource intensive, the PC's hardware can make a considerable difference. For complex editing a gaming-class machine is often recommended.
https://www.desktop-documentaries.com/computer-for-video-editing.html
Lengthy projects with high-res files and many effects that tend to hang on export may benefit from being produced and exported in segments, later to be merged for final export.
