You need to go to a corporate store (not a Verizon authorized or premium retailer, never go to those) and tell them to file a Device Management Database (DMD) form to whitelist the IMEI of the device. They need Tier 2 Tech Support to do this. If a rep says otherwise, tell them you need to speak to a manager. Also, it is not regular customer care, they are generally not knowledgable, it is Tier 2 Tech Support that needs to file the DMD form.
The Verizon, Sprint and SIM free iP7/7+ are the same identical hardware/firmware. The only issue is whitelisting the IMEI. This happens EVERY YEAR. Verizon delays whitelisting the IMEIs of non-Verizon iPhone models. They eventually do it after people start complaining to the FCC. You can file a complaint with the FCC as Verizon MUST BY LAW allow you to use a Sprint or SIM free iP7/7+ on their network. However, hopefully you will encounter a rep who isn't an idiot and understands about DMD forms and whitelisting of IMEIs.
It takes about 2 business days for the IMEI to be whitelisted.