While it goes without saying that posting frequently is a good idea you don't need to always post content to all of your followers.
Instead you can use Instagram direct to target a specific group of users by privately direct messaging them a photo or video.
Although this is a little more time consuming the benefits in terms of engagement and potential increase in followers are worth the effort.
As an example, if you were to run an in-store activity or Instagram live session after the event you could post a specific image or video just to the people who attended.
Maybe you could also add a link in your Instagram vape shop bio to a special offer