The VP has no role of significance other than breaking ties in the Senate. He has zero constitutional authority or portfolio of his own, which is why the "bucket of warm spit" comparison was made. Other than that, he is completely dependent upon the President to assign him anything of substance. In other words, a President can easily sideline the Veep completely, and render him a complete irrelevancy.
Cruz would be a fool to make himself that vulnerable to the caprice of Trump
I agree that on the surface, it would not appear to be a good career move for Senator Cruz. However, if he expected Mr. Trump to lose, it may be a good move. Also, Mr. Trump would have no choice but to accept him as a running mate or to resign the nomination at which point, rule 9 would apply and the RNC would appoint a replacement. While the initial discussion was about the possibility of Senator Cruz as the vice presidential nominee, the same consideration would apply to Gov. Bush, Gov. Romney or any other person the delegates wanted to select as the vice presidential candidate.
This is all just speculation as the likelihood of it happening is quite remote. Never the less, it is nice to have this period of consequence free time to speculate while heightened emotions are allowed to cool.