Traditionally that may be the case, but this year might be different since Biden is clearly a walking corpse and anyone with even the slightest degree of awareness is well aware of that. Add to that Trump's recent covid health scare, the VP issue has become critically important, much more than in "normal" election years. 2020 is far from normal, in many ways.
Now that the debate is over the battle about who won begins. Fox and talk radio vs. everyone else in the media world. Each side will offer up snippets and intone authoritatively about their significance.
And I think you are correct, this vice presidential debate is out of the ordinary for the reasons you expressed, the question is whether it will break the mold and actually have much effect?