Please don't do this. Just because I've got a large viewport, it doesn't mean I want massive text, and just because I've got a small viewport, it doesn't mean I want tiny text. Either extreme can result in very frustrating to read and potentially inaccessible text. Perhaps it makes sense in some narrow cases (e.g. the main page title), but certainly not for your body copy.

With calc() you can get some pretty decent scaling based on vw. If it was just a direct proportion of vw, then yes, things may look a bit on the larger side on a bigger screen, but as with a lot of things, if used with care, the results are pretty nice.

It's not something that would be done as the only thing that influences the final fonts size. As another comment pointed out, it could be combined with calc(). But also, within media queries, to achieve reasonable results in various widths while still responding in a subtle way to the actual viewport with of the client.