Just as a data point for anyone else attempting to use Virtual Post Mail for credit card churning purposes - for the first time I got an error message when using a VPM mailbox on an application. Amex told me directly in an auto-generated email that the "address is a mail service center." This is the first time that any credit card company has figured out that it's not a real street address...

Tried to keep going with VPM after returning to the States since I'll still be moving around every few months and thought it would be easier to keep a single address for credit-related purposes. But, given that I'm not foreseeing long stretches of time abroad for the foreseeable future, I might just save a hundred bucks or so a year and start using my parents as my mail forwarder...