Right. A Neder and a Shevua (unless it is Al Daas Acheirim) does not happen if you don’t mean it.
Sam2,
Actually the Beis Yosef was not sure if fasting Erev Yom Kippur might be a Mitzva, too. The Lashon of the Gemara is that eating Erev Yom Kippur is as if you fasted both days. That might mean that fasting is even better.
However, the Tur and Rashi explain that the Gemara means that it is as if there is a Mitzva to fast and you fasted.