One way to do this is with calc. As far as i know, 100% is the width including scrollbars. So if you do:
body {
width: calc(100vw - (100vw - 100%));
}
You get the 100vw minus the width of the scrollbar.
You can do this with height as well, if you want a square that’s 50% of the viewport for example (minus 50% of the scollbar width)
.box {
width: calc(50vw - ((100vw - 100%)/2))
height: 0
padding-bottom: calc(50vw - ((100vw - 100%)/2))
}