By three weeks I know the boys from the girls by comb size and action. One we knew right after he was hatched. This l;ast hatch I new by the time he was 2 days old. Although some leave me hanging. My little cochin I posted last night and all said girl
As far as vent sexing you really have to know what you are doing you can easily hurt a chick doing it, and from what I have read you should never try to vent sex a bantam. The hatcheries dont even do it, usually anyway.
There is no safe and reliable way to sex a baby chick (unless you're a professional vent sexer who has gone through tons of training). You'll see all sorts of old wives tales - from people telling you that you can check their vents, their wings, hold them upside down, see which ones are more aggressive, look at their tail feathers, etc. But none of these methods work reliably.
You just have to wait... in a few weeks you'll be able to tell.