In 1988, I was supporting Senator Paul Simon for the Democratic nomination, but the supporters of Governor Michael Dukakis made the argument about executive experience. That's obviously not the only reason he won the nomination, but I'm sure it helped. Of course, in 2004, Governor Howard Dean lost to Senator John Kerry, and in 2012 Governor Bill Richardson finished behind several Senators, so I'm not saying it's an automatic formula for electoral success.
As for being a good President, there's also no automatic formula. In 1988, we in the Simon campaign were pointing out that a Senator acquires more familiarity with a wide range of federal issues, such as foreign policy, that a Governor just reads about in the paper, and the same could be said of Sanders as compared with O'Malley.
With regard to Truman and Johnson, their VP experiences don't matter much, because at the time each won his sole Presidential race, he'd been President for a while. Better examples would be the current or former VP's who hadn't been President (Nixon, Bush41, Gore). The extent to which they were in the loop might not be known, or even considered, except by politics junkies. For millions of voters, they had the image of having been in an executive position, instead of just voting Yea or Nay on bills (which is the oversimplified image many people have of what Senators do).