The default theme function for all fields is (wait for it),
theme_field(). You can copy this function into your own theme, and rename it to
MYTHEME_field(), either to alter or to refer to as reference.
You would then name your specific version
MYTHEME_field__field_video_virtual_tour__basic_drealty_listing(). Note the two underscores separating the parts.
For reference, the standard suggestions are
Also remember, these are really just to wrap your elements with some HTML. If you want to change the way the element itself gets rendered out, you should really make a field formatter.