Here's Marco Rubio on Fox News last night, after Sean Hannity asked if Marco felt he had an "obligation to at least leave that door [VP nominee] open."
"I'm very honored and privileged to serve this country in the United States Senate and to represent the state of Florida in the United States Senate.
That's where my heart is, that's where my mind is, that's where my focus is. I don't believe I'm going to be asked to be the vice-presidential nominee. That's not what I intend to be, that's not want I intend to be, and that's not what's going to happen."
Here's a hope that people don't focus on "that's not what's going to happen", and read the 74 words before that where he sounds as open as anyone can be.
There's no promise at any point, to turn down an offer; there's merely a prediction that he won't be the nominee.
Btw, if you missed it yesterday, read my post about the history of Rubio's responses to the VP question. It's not nearly so unequivocal as often portrayed.