Welcome to the boards. To help answer your question I can give you my take. I have never heard of a BVG holder being tampered with as far as switching a card out. I have seen the older PSA holders that have been tampered with. The new PSA holder is tamper proof to the best of my knowledge. As far as the SGC holder I am not sure about. That is my 2 cents. I am sure others will have input on this subject.
Here is a recent thread on a Mantle card that might interest you.
http://www.net54baseball.com/showthread.php?t=214067