When using the Vendor Modifier on a vendor that had email settings configured, those settings are erased after using Modifier to change the Vendor ID. After processing your change, you need to go into the Vendor Card, click the Email button, and then reset any configuration of email settings for the vendor(s) in question.
I didn't test the Customer Modifier, but I assume this bug exists there, as well.
If you would like Microsoft to add this to their list of fixes, please vote on the suggestion here: https://connect.microsoft.com/dynamicssuggestions/feedback/details/1008370
*This post is locked for comments