I have never tried vent sexing, but I have spent a good amount of time researching it. I would approach it cautiously. It is easy to injure chicks if it is not done properly. It is a risk for newbs to try to train themselves. So if you have enough chicks that you don't mind losing a few as you "practice," then it might be a fine idea. But if not, you're better off waiting a couple weeks and looking for tell-tale signs as their combs start to develop/turn pink and they start to feather out. I find that as long as I have a group of the same age, same breed of chickens, the cockerels are usually easy to pick out by their combs starting at a surprisingly young age.
The other option is to sell them straight run at a very young age.