I don’t know what the time frame would be to let public knowledge of the choices for VP out.....
The article was about how few Congresscritters have endorsed Trump. It was talking about how the obvious to some choice to endorse Trump was Sessions since he has been mentioned several times by Trump, but the article quoted Sessions as saying it had been a while since he had even spoken to Trump.
Now as you probable assume (and I agree). A VP has one job, basically to shut up and not stir too many waves until being announced. So one can't really read too much into the silence thing. Trump may be secretly thinking of asking him, or not. Sessions is not one to really lie about direct questions about things, so I am torn what to think.
As a Cruz supporter, I am hoping the silence is that Trump just used Sessions in the immigration debate as a pawn to help his campaign but really doesn't like him, so he will end up supporting Cruz. But Sessions is no dummy, he may be just waiting to see which side is winning. There is no rush for him unless he is pushed. Perhaps he was pushed and declined to give in ticking one side off (we know who has a temper). Who knows. Thanks for listening to my speculation.