I am not sure the VP selectee really helps the top of the ticket, but he/she can sure cause it problems.
Remember Perot’s selectee Stockton? After that VP debate, many viewers were questioning Stockton’s mental capability, should he become president. Stockton did not perform well in that debate and it cost Perot votes.
Just another example of Trump tearing up the standard "play-book" in the way he conducts his campaign.
The Stockton - Pence comparison is like comparing Ross Perot to Donald Trump.