Unlocked and Verizon should not be placed in the same sentence
Actually any phone that runs on Verizon's LTE bands has to be unlocked due to FCC regulations on the block that they own. Technically they could activate and work on Sprint's network, however Sprint won't have the Hardware IDs in their database and will not be helpful at all.
That said there are some phones that Verizon has sold before that included GSM radios. Hell even the latest CDMA iPhone I believe has support for certain GSM bands. Correct me if I am wrong though.