I don't think the claim is as bold as you make out. See Kevin Nisbet...
Vaughan certainly displayed more potential and talent at a young age than the aforementioned, just see Nisbets history pre Raith Rovers to prove that.
Far from certain that he would have got a sniff at the scotland squad, but when you see some of the players that have in recent years ( Brophy, Burke, Shinnie, ect), it's certainly plausible that no injuries and the premiership move he would have got would have spurred national team discussion, given a good bit of form.
If your nan had wheels and all that however...