This is a thought, but I wonder if the recipient is replying to your texts multiple ways (like sometimes from the phone, and sometimes from email). If they are set up to get the texts in their email as well and they reply from email, it might come in that way rather than if they reply through the phone's messaging app (the vtext address is for SMS and the vzwpix address is for MMS).
I am not sure on that though. I'd have to run some tests.