Laptop batts are more sophisticated. If you've ever opened one, you'll find a lot of electronics in there along with the cell pack. A laptop actually switches over to the external supply when available and isolates charging from loading. Cell phones do the same thing.
In any case, it shouldn't be an issue to load the pack while the balancing charger is going. As already mentioned, the charger will just run up to the current limit. It will provide some of the current powering the atomizer and the pack will pick up the rest of the load.
The only issue I can see is if the charger has any safety features that could get confused by the sudden change in loading. It that case, the charger would probably just shut down with some kind of fault indication. Depends on the charger, you'd have to try it and see.