Is it unrealistic?
In a very, very basic sense, you can think of a stealth fighter/bomber as an aircraft that will be detected at about 1/10th the range that you would detect a 'normal fighter' at. If the Vega can pick up a basic fighter 3-6m^2 at 300km, then there's nothing strange about it picking up an F-117 at 30km.
If, however, the radar is also being jammed, then it may be operating on a lower gain, so that range shrinks even more.
And this leads us to SAMSIM unrealistic (what-if) scenario with Vega in Serbia :-)