I'm on a family plan with someone who actually has the tethering plan with Verizon. (I do not) If I use their phone number for the user name to login on my tablet instead of mine. (their#@vzw3g.com instead of my#@vzw3g.com)

Can I get internet without a huge $1.99/MB charge on my bill? (Internet with no additional charges from what we have now)

Would I also need to use their number when I do the dun hack on my phone, or would it work with my number? Would both phones be able to use it at the same time or only one? And yes I have the permission of the owner of the other phone to do this.

Has this ever been tried?

Wait. Are you trying to connect VIA their phone to the internet (aka its possible), or share some other phone's internet through your phone to the tablet. That is impossible. The data connection is tied to the SIM card of the phone, or something similar. I don't think its possible, but you can try...

I am using Verizon so there is no Sim card involved. I'm trying to use my phone with another phone's tethering plan, I thought because I would be using their number instead of mine to authenticate it would look like it was on their phone, or at least not charge me data.

No, they are not that stupid. Verizon uses even "worse" tech than SIM. If you had SIM, then you could just swap the SIM and you can have his/her data. They tie your phone number with the physical phone, so it is going to be very very hard to do what you want (aka impossible except for Mr.SuperHaxor)

Then just wondering, if I did do this what would happen? Would it would just get charged to my phone?

No, I think it would just give an error. It should not even work... If it does work, then watch your bill VERY closely for some time.