I think native English speaker who had never heard of Vaughan (sure we can find some of those) would likely to pronounce it like "Vog-un" - /ˈvɒɡən/ or "Vog-han" - /ˈvɑːɡən/

This sent me down a mental rabbit hole, I think it's one of those interesting nuances that are rules that native speakers follow without being able to name it, or know it. I'm a native speaker, and also thought `vawn` was the most obvious pronounciation. I'm guessing it's because `augh` is perceived as a recognizable vowel cluster where `gh` tends to be silent (daughter, caught, naught, taught). The interesting twist for me is that `laugh` is in obvious counter example, until I realized that gh in final position (laugh, rough, enough) is almost always \f\. And further, in words like laughter, roughness, we immediately distinguish a modified root word from the lexical position.

Maybe there's also an interesting thread to pull on in that the pattern may be more pronounced for names (e.g. Hughes). Just ruminating here though, I don't have a source for any of this.

No, I don't think they would. I've never heard of Vaughan and assumed one syllable like the parent commenter.

No, "gh" is usually silent in English spelling.