iMessage problem/email.?

Okay, so let me start off by saying that when I text people, I text them off of my email. I learned a while ago that if you add "@vtext.com" to the end of the number. However, lately for some reason, I have not been able to do that to new numbers. I can only message contacts like that which have already been in my contacts. When I send a message to phonenumber@vtext.com, some numbers come back with phonenumber@vzwpix.com" What happened? Why is their address different than when I sent it and how come they got it o begin with if the addresses are different? Also, when I message some people (that have an iPhone), I can't type in their number and add @vtext.com, it HAS to be @vzwpix.com. When they do they back, it doesn't come into my mail, but rather my iMessage. I want to get all my messages in one place and using one suffix. What should I do. Is there a way I can select specific contacts so that I can receive their texts through iMessage? Thanks.