As @DanBlake had mentioned, there is a serious FTC issue where individuals that were casually linking to a merchant are now unwittingly becoming advertisers. There's a decent snippet in the comments of the above link about that.
They claim that they are just turning non-affiliate links into affiliate links.
They also claim they are not overwriting other affiliate links.
So do they automatically recognize all other affiliate network redirects? What about direct linking affiliate programs? What about Pay-Per-Call programs? There are a number of ways this can go wrong and take money from other affiliates by hijacking the links.
Not to mention the fact that there is very little benefit to the merchant here - that is traffic that quite clearly they were getting anyways. There is complete intent to link to them, so why must they pay VigLink and Bit.ly for shortening the URL? That's a service being used by the individual linking to the merchant. It's not worth commissioning on, there's very little value to the merchant.
If you have an affiliate program with Viglink, please contact them and request they remove ALL URL shorteners from their program - it's not just bit.ly, it's tinyurl as well, and a couple others that I cannot recall at the moment.