So according to Reddit - you can indeed buy it from Apple... See here:
https://www.reddit.com/r/verizon/comments/53dzix/650_iphone_7_trade_in_promo/d7smsgq
I'm wondering about this also but I've not heard a lot of people saying they have done this successfully in the Verizon preorder thread and I've read some people were told by Verizon that it had to be purchased at a Verizon store or authorized dealer. I'm sure Verizon will take the phone in trade but whether they give you the $650 credit over 24 months or just their trade-in value hasn't been definitively established. I would think that more people would have done it with the delivery delays at Verizon if it were possible.