Read the fine print. I started to do something similar from a different place for my GNex. After waiting a month after release, I canceled it simply because I didn't want to wait any longer.
Fine print with Wirefly:
When you sign up for wireless carrier service through Wirefly, we discount your phone purchase price by an Equipment Discount amount ranging from $200 to $300. By ordering through Wirefly, you agree to repay this discount if, during the 181 days after your new equipment is activated, if you:
* Fail to pay your balance due to your carrier each month
* Disconnect this line of service
* Transfer this equipment to another line of service
* Change your carrier service rate plan to a lower monthly service rate plan
* Deactivate the email/data feature if one was required at the point of purchase
* Transfer/port an existing phone number from another account if this is a new line of service
* Replace an existing account with this account if this is a new line of service
* Return or exchange your phone and it is not in its original, new condition with no more than 30 minutes of usage
The equipment discount. The cell phone that we offer free or at a significant discount may have cost Wirefly several hundred dollars or more. Since we are only able to offer you such great equipment discounts because of our commission-based business model, when you order from Wirefly you agree to be charged the phone?s full cost if the carrier reports to us that you broke your cell phone contract, or if you return a phone and it?s been damaged in some way. This protects both Wirefly and the carriers from fraud, and allows us to continue to offer the great deals we are known for.
My understanding is that last part is separate from the ETF you'd pay Verizon. Saving a $100 may not be saving $100 in the long run.