You can swap the field order within the right-click menu to Events - Media tab.
Also there was a script once published in our (german) Vegas forum which did correct all Events properties which are indicated as Upper Fields to Lower Fields.

Furthermore - if you wanna do this because after a render or after import of some kind of media Vegas seemed to swap the correct field order - modifying the "vegas profiles.ini" file will help to overcome this problem right from the base.

Marco