[susan53]

So = an emphatic word. It's used in the sentences you quote in order to be emphatic (and is usually stressed). Compare :
It was so hot!
It was really hot!
It was incredibly hot.
It was extremely hot
It was very hot.

They're on a scale from emphatic (so) to non-emphatic (very). Intonation can of course affect the degree of emphasis. Heavy stress on "incredibly" would make it more emphatic than an unstressed "really" of course, so the positions on the list aren't rigid. But intonation aside, "very" is definitely down the bottom.

As for your question - as always it depends on the context. It is impossible to say yes or no without knowing the situation. So eg here it would be fine :

A : I was very nervous before the interview.
B : You were very nervous? Why were you very nervous? You were the ideal candidate!

But in other contexts, eg where the speaker is irritated by the other person's nervousness, "so" makes more sense - both for the degree of emphasis and because it has the meaning "to the degree that I can see", which is right for the context.

Will you please stop pacing up and down! Sit down and relax, for heaven's sake. Why are you so nervous? It's only a dentist's appointment!