So, there are a few things to assess here.
First of all the Chasuble says “this dude’s value cannot be reduced.” Just like the previous announcement on Fancy New Hat, this will only prevent value reduction on the dude while he is wearing it. It does not cancel previous reductions. If the Dude was hit by Sanitorium and then receives the Chasuble, he still has the -1 value until its normal “end.” (Sundown)
Second, it says “conditions cannot be attached to this dude.” This is a prevention effect that keeps future conditions from being attached to the dude. It does not remove previous conditions.