content section below content section

you see your example page – that section scrolls away!
but you set your video container to fixed position:

#video-container {
  position: fixed;
  top: 0;
  left: 0;
  width: 100vw;
  height: 100vh;
  z-index: -1;
}

so this will stay at position

do you like to have this?

#video-container {
  position: absolute;
  top: 0;
  left: 0;
  width: 100vw;
  height: 100vh;
  z-index: -1;
}

#top #av_section_1 .container {
  max-width: 100vw !important;
  margin: 0 !important;
  top: 0;
  left: 0 !important;
  position: absolute;
  padding: 0 !important;
}

this is the code for your vuur4 page now:

#av_section_1 {
  height: 100vh !important;
}

.responsive #av_section_1 .container {
  max-width: 100%  !important;
  padding:0 !important;
  margin: 0 !important;
}

video {
  position: absolute;
  top: 0;
  left: 0;
  width: 100vw;
  height: 100vh;
  z-index: -1;
  object-fit: cover;
  overflow: visible !important;
}

ps: you can shift the video position by

/* === Keeping the fireplace in view === */
@media only screen and (max-width: 1425px) {
  video {
    object-position: -335px;
  }
}

I’m on a Retina display here and can’t test it under all eventualities – maybe you needed to remove that rule set:

@media only screen and (max-width: 1425px) {
  video {
    object-position: -335px;
  }
}

and find your own media-query setting for it maybe with more conditions like -webkit-min-device-pixel-ratio: 2 or min-resolution: 192dpi

maybe we start with :

@media only screen and (max-width: 875px) {
    video {
        object-position:-235px 
    }
}