I don't think ONE thing would be enough because there are two problems with the Vita: Software and Memory. It sounds controversial but I would have launched the system at $299 with 32GB of internal memory, that price is high but this is the initial cost everyone thought the system would be, I think people would have been fine it, its the price of the memory cards that put a bad taste in peoples mouths. This way the memory cards could remain proprietary but would be completely optional and only available in $16GB ($39.99) and 32GB ($69.99).
In terms of games, any one of these changes would have made a big difference.
-Final Fantasy Type 0 as a launch window title, slightly upgraded visuals from the PSP version and released both in Japan and West in summer.
-Jak 4 instead of Uncharted Golden Abyss. Uncharted is a nice tech demo but does anyone really want to play a watered down version of the PS3 games?
-Elderscrolls Morrowind remaster exclusive to the Vita with a huge graphical overhaul.