The false positive rate is going to be insane. You mentioned roaming, but there are so many other scenarios where this could trigger - so, the user sits down at a starbucks and suddenly can't access the client's webpage, with some very confusing error about VPNs. Guess what, they are not going to fix their network, they are going to give up on going to that website. Without a plan how to measure/fix false positives, there's no way this would last more than a week in a real e-commerce environment, unless there's already a dramatic problem with VPNs (like at Etsy).