I will try to explain it better.
Technically there is a vent. It is small and you would have to forcibly open your mouth to open that little vent. I'm sure it has more to do with keeping someone from suffocating than actually allowing mouth breathing. The point of the strip is to NOT breathe through your mouth. However if you could not breathe through your nose, by forcing your mouth open, it would open the vent. The adhesive is strong.
I press my lips firmly together and then apply the strip. I have never had a leak or the vent open. The vent is so small that, even if it were open, your machine could easily compensate.
My problem with skin tape (trying to use cheaper methods) is that I develop an annoying leak by 3 am. The adhesive is not strong enough. But because it touches my face, not just my lips, I don't want the tape adhesive any stronger. It makes my skin red and even makeup can't hide the redness.
Somnifix strips leave no redness, no latex allergy, and I can happily continue using my nasal pillows. They work for me.
I don't want to sleep like a baby. I want to sleep like my husband. Resmed AirSense 10 for her. Pillows. Humidity 4.