If you're taking IDPA seriously, a concealment vest isn't needed. What is needed is what you would NORMALLY wear when carrying concealed. Anything else is "gaming."
How many folks wear a vest when not at an IDPA match? Nowadays, a vest is a flag that says, "Hey, look at me, I'm packing!!!"
Most of us, when we're shooting from concealment, just wear the jacket or shirt we'd normally wear.
You can change what you'd normally wear to make drawing from concealment easier, and that's the ethical thing to do. It also makes good sense.
From my perspective (and my perspective alone), the primary objective of shooting IDPA is to hone your self-defense skills; only secondly is the objective to do well in a match. When the second objective outweighs the first, you're either very, very good (master class improving already very solid skills) or your fooling yourself about why you're really out there.
I can understand why others might feel differently, and they shoot the same courses of fire I do -- and also have opinions -- so, I agree with UT83 on this one: to each his own.