The Vita doesn't update in the background like the Wii U does so the situations are not comparible. If it asks you to update, choose no. If you accidently choose yes, turn your machine off before it finishes downloading the update.
The Wii U literally downloads and installs the updates in the background, even in sleep mode. The Vita doesn't. If you're that worried, turn off wifi and only turn it on when you need it (to reinstall Henkaku or whatever).
Perfect, thanks for the clarification