Basically the answer is no, if you have a vertical scrollbar there is no way to make 100vw equal the width of the visible viewport. Here are the solutions that I have found for this issue.
warning: I have not tested these solutions for browser support
tl;dr
If you need an element to be 100% width of the visible viewport(viewport minus scrollbar) you will need to set it to 100% of the body. You can't do it with vw units if there is a vertical scrollbar.
1. Set all ancestor elements to position static
If you make sure that all of .box's ancestors are set to position: static; then set .box to width: 100%; so it will be 100% of the body's width. This is not always possible though. Sometimes you need one of the ancestors to be position: absolute; or position: relative;.
Example
2. Move the element outside of non-static ancestors
If you can't set the ancestor elements to position: static; you will need to move .box outside of them. This will allow you to set the element to 100% of the body width.
Example
3. Remove Vertical Scrollbar
If you don't need vertical scrolling you can just remove the vertical scrollbar by setting the <html> element to overflow-y: hidden;.
Example
4. Remove Horizontal Scrollbar
This does not fix the problem, but may be suitable for some situations.
Setting the <html> element to overflow-y: scroll; overflow-x: hidden; will prevent the horizontal scrollbar from appearing, but the 100vw element will still overflow.
Example
Viewport-Percentage Lengths Spec
The viewport-percentage lengths are relative to the size of the
initial containing block. When the height or width of the initial
containing block is changed, they are scaled accordingly. However,
when the value of overflow on the root element is auto, any scroll
bars are assumed not to exist. Note that the initial containing
block’s size is affected by the presence of scrollbars on the
viewport.
It appears that there is a bug because vw units should only include the scrollbar width when overflow is set to auto on the root element. But I've tried setting the root element to overflow: scroll; and it did not change.
Example
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