You should be fine opening the airlock. Because the wort puts off CO2 and it is heavier than O2, your wort essentially protects itself from oxygenation. The airlock will keep other dirt, bacteria, etc. from getting in. Might want to find out if you have a leak somewhere with your airlock. A teaspoon or so of vodka in your brew is no big deal, but you don't want to compromise your hard work if your leak is more substantial.