Due to the way the Vaccinator was changed in the 2015 update, the Über Duration upgrade in MVM does nothing to the actual duration of the Über.
This is now fixed using a server side mod. Each upgrade now adds an additional 0.5 seconds per level, applied to each individual Über charge for total up to 6 additional seconds per full Über meter. And up to 4.5 seconds to each individual resistance given, from the original 3 seconds.
Big thanks to Shylo for pointing this out to me, as I had absolutely no idea!