I'm not in the know so this is 100% speculation.
I very much doubt Verizon is going to get a slider Nexus Prime (or whatever it is going to be called). First of all, Verizon, as you have already pointed out, has a line up of slider phones already. They already have the Droid 3 which has sold quite well and soon they will be getting the Stratosphere. Considering that VZW tends to stray away from sliders, besides their Droid line, I would find it highly unlikely that they would add another so quickly. The fact that VZW is getting the Stratosphere, a samsung device, in October leads me to believe that we will not see the Prime as a slider.
My second reason is that we have heard no rumors of a slider coming to VZW. In fact there has been no mention of it anywhere from what I can tell. Many writers, like Kellex at droid-life, have come out and said that what was mentioned in the tweet was not the same phone as the one we are all thinking of. Right now it seems like Kellex knows more about the device than he is letting on. He seems pretty adamant that he is right on this one.
Lastly, there is no way that a slider will become a flagship device. There has not been a single slider flagship device on any network since the OG Droid. On top of that, even after the success of the OG Droid all sliders have on every single network have been mid range devices. Market patterns seem to indicate that consumers want choices, which is why we have sliders, but that a vast majority of them want them without it.
Of course I could be horribly wrong and the VZW device will be a slider, but then again this is all speculation anyways.