Agreed, the reason KW traded for Vazquez was because he didn't think that he would be able to keep either Garland or Contreras long-term. He thought he may have to trade one of them. That means he probably only expected to have 5 starters, not 6. What if 2 of our starters get injured? Should we keep 7 starters then?
And I think people are expecting far too much for Contreras in a trade. Even if you believe that he is a top pitcher in baseball because he had one great half, he's still only under contract for one season. Expecting to get one of the best prospects in baseball in Milledge, Duaner Sanchez, AND another pitching prospect that could start for one season of Contreras is pretty ridiculous.
I guarantee that people will be pissed if we don't win the World Series next season and Contreras then goes on to sign elsewhere in the offseason. Everyone will say, "WHY DIDN'T WE TRADE CONTRERAS?!?"