Yup, the suck back effect. When my beer is fermenting and the temperature is changing, like when I begin to cold crash, I remove the airlock (keep the bung piece in) and cover the carboy opening with 2-3 paper towels soaked in StarSan. This keeps the sanitizer/vodka out of my beer and adds a layer of protection to the beer. That's my approach anyway
Don't worry too much about the vodka though, it's really a small amount in a large batch of beer, I doubt you'll even know it's there.