Most vapes cannot balance charge more than one cell. One cell is charged and then after that the next cell is charged. All of the drain, excess heat and over use of that one cell causes strain on that 1st battery in the vape and ultimately leads to the demise of that 1 battery leading to the need of a new pair. If lucky it will just stop working if unlucky the cell may go into melt down
If your batteries are not the same age, ie the same amount of charges, the same amount of use and charged together for their whole lives then they are unbalanced. Meaning they cannot deliver the high output required by your vape. This will lead to low performance and low battery life.
External chargers let you monitor the charge going in to your cells. Good quality chargers will put what is needed in as opposed to as much as possible as fast as possible which can caused issues. A good charger will slow the charge rate down to recondition a bad cell. Internal charging will not do this. Using a good external charger could mean your batteries getting you through a day away from your charging equipment or not. Ultimately it is up to you how safe you want to be. Yes an external charger may sound like a hassle to you but it depends on the risks you want to take on safety and performance.