By my observation, the VP job is just a glorified lackey to the president. The prez assigns the VP jobs he doesn't want to jack with. The qualities needed are someone who can think & talk on the fly & take no sh*t. As for adding to the elect-ability of the prez, they need to be personable & pleasant in appearance (women good looking). As to the possibility that the VP may have to take over as POTUS, voters could care less.
With only four years of eligibility, Trump's VP will be well set to become the next president, I believe that's what most voters will be considering.
