Vaper is superior against Battery Mod because Vaper gives Steam Power and Reduce Battery Consumption while Battery Mod only give Battery Capacity. Reduce Battery Consumption and Battery Capacity are just the same because 1 bar of Reduce Battery Consumption is equivalent of 1 bar of Battery Capacity. However Battery Mod has twice more or less "Battery" value than Vaper but Steam Power is more important! Steam Power indirectly improve your Battery Consumption/Capacity because more power means less vaping needed specially on levels that has time limit, wind or an opponent vaper. Steam Power in Vaper also helps you in any type of levels while Battery Mod doesn't help you in levels that has time limit.
There are five types of categories found in the shop. These are Style, Vaper, Battery Mod, Accessories and Stickers. Only the Vaper and Battery Mod helps you in-game, the other three are useless and offer's nothing useful in-game. Style, Accessories and Stickers are just visual aesthetics for your character. It's a fashion thing that doesn't improve your Vaping Skills of your character. Some of the items are very expensive! You need more money to upgrade Vaper and Battery Mod if you want to improve your character and beat difficult levels.
Since Vaper is superior than Battery Mod, it doesn't mean you need to buy newly unlock vaper every time. Money are limited in terms of upgrading Vaper and Battery Mod. Fortunately Vape Master Game doesn't have energy feature or any similar feature that limits playing time. So if you want to earn money, just play the game as much as you can. But it will be difficult to earn money if you buy everything even if you avoid buying useless items. It still needs money management to avoid wasting time and money, select the best items to buy and you will beat levels easily.
The simple trick to save money and buy efficient items is to buy only when the current level is very difficult. Push your self to beat a current level without upgrading, if you know that the level is still impossible to beat while performing good and awesome vape or flawlessly avoid being caught by a guard, then buying the current best item unlocked is needed. Only buy battery Mod if you have the current unlocked best vaper item. A good Vaper is enough even if you don't have the best or good battery mod. This trick will help you save money to buy future items and unlock missions.