The way the word is used in Hebrew isn’t that meaning of vain. It is vain in the sense of “vanity.” Emptiness of speech, lying, etc.
This. The word used in the original Hebrew is “shav” (שוא) - which can mean either “vain” or “false” - and the original meaning of the commandant is to not swear in God’s name on something that’s not true. Of course, the religious leadership immediately thought “well, we’re obviously not going to refrain from lying, and refusing to invoke God’s name just because we’re lying is going to make it suspicious, so let’s just say we are not supposed to invoke his name at all”
This. The word used in the original Hebrew is “shav” (שוא) - which can mean either “vain” or “false” - and the original meaning of the commandant is to not swear in God’s name on something that’s not true. Of course, the religious leadership immediately thought “well, we’re obviously not going to refrain from lying, and refusing to invoke God’s name just because we’re lying is going to make it suspicious, so let’s just say we are not supposed to invoke his name at all”