I'm sorry, the ideological classification of vice-presidents that you have posted is completely absurd. How exactly is John Nance Garner the most liberal vice-president on that list?
What part of the data do you take issue with? Those ideology scores aren't just pulled out of thin air. They're DW-NOMINATE scores which are used in political science as the gold standard for comparing the ideologies of legislators. Garner's score there is an objective measure based on his voting behavior relative to the rest of his congress (taking into account every roll call vote in a given congress).
DW-NOMINATE does a very poor job of comparing people from different eras. Also, they may label their axis as being Liberal-Conservative, but it really is more of a Democratic-Republican axis than Liberal-Conservative.
Hence the argument that the chart is a useful measure of partisanship rather than anything else. I think Silver's point regarding Ryan stands if we're talking about partisanship rather than ideology per se, though.
In that case, Ryan is the vp choice who was least likely to vote against his party.
It's more accurate to say he's the least likely to vote with the other party - which isn't the same thing (see for example, Ron Paul's extreme score while he votes against the majority position of his party all the time).
