I have the same configuration and I ran into the same problem. I contacted Vibe Commerce and they fixed it for me, although I had to pay for it. I think the code below was the simple fix, but not 100% sure - worth a try. If it doesn't work, contact Vibe Commerce's support.
This looks like the fix, but I don't have the original file handy to compare. I found this code in the newminicart.js file in my App_Themes/Skin_1 folder - check your code and see if it is different as I believe this may be the change that was made to fix it:
In function ajaxifyAddToCartForm(form, productimageid, productID, variantID)
Code:
//var oldwebformsubmit = WebForm_OnSubmit;
var oldwebformsubmit = (typeof WebForm_OnSubmit === "undefined") ? 0 : WebForm_OnSubmit;
var oldformsubmit = form.onsubmit;