I also hope that this is not the reason that Dick Cheney is not at this event, hopefully he either had something else schedule or maybe intelligence is still that it is not advisable that they are seen together in a public setting.
They hardly ever appeared together at events outside the DC area after 9/11 and then it was very official events or events where it was expected that the President and VP attend.
I tend to believe it is something do do with a prior engagement or security rather a quarrel because we all know that in politics many people will appear with others even if they disagree for political reasons and a reunion I would have thought would have been in that category.