The FIRST thing the Senate needs to vote on is whether the 2 Articles are constitutionally flawed on their face. If the Senate votes no, they are not valid, it ends right there. It never gets to a vote on witnesses or guilt on the articles. To call witnesses on invalid articles only serves to validate flawed articles.
The only difference between us is that I want a vote on just the validity of the articles. Any other votes like witnesses or acquittal validates the process.