This thing is good. I like seeing the traffic numbers displayed right there on the extension.
I'm not exactly sure where all the data comes from. I have noticed for many large sites the numbers closely follow Similarweb, but Vstat actually provides numbers for many smaller sites that Similarweb doesn't even provide ANY data for.
Like all of these tools you have to take the info with a grain of salt, and some of it doesn't seem to make sense sometimes ... but, it is still highly useful if you're aware of the limitations.
Hopefully it gets even better in the future (more data is always good :)