I noticed that the account name (that's what it's called on my Mac) where you put your phone # string in the Bluetooth panel to enable tethering with VZW seems to be a little different when I see someone post as to what the parameters are to make tethering work. I've seen "email@example.com",
", and a few others. I tried both of these and they both worked.
So after some experimentation, I've come to find out that it doesn't matter what you put after the "@" sign, it always works. I put several random strings of characters and they all worked. I now have mine setup with nothing after my phone number - not even the "@" sign - and it works.
My guess is that under the modem tab in the Advanced options, where you tell it to use Vendor "Other" and Model "Verizon Support (PC 5220)", that makes it default to whatever's after the "@" sign.
Again, I'm on a Mac. But I think it probably works the same basic way on Windows, but the setup screens and how you get to things is obviously different.