Okay I'm assuming that i can tribute Light and Darkness Dragon for Eradicator Epidemic Virus. When I tribute LADD it is the cost of activating , EEV and since he cannot costs. When he leaves the field EEV will resolve properly. Also im referring to a full strength LADD, just summoned for example.

Is that correct?

Yes, the tribute is a cost...and LaDD cant negate himself being used as cost.

Also works for DDV.