The business cards are smaller than the standard British kind and incredibly flimsy.
I don't think there really is a "standard" size any more with each printer designing their own templates in sizes they see fit.
In the old days (when type was a set size and couldn't be stretched to fit any available space - yuck!!!) business, or visiting, cards came in a variety of standard sizes including thirds, extra thirds, reduced small and small. Also available were single and double panel cards which were pre-embossed.
I would say, if you are going to use Vistaprint, step up from the "just pay postage" version. Use a minimum of 300gm but 330gm would be better. These cards are so cheap now that you should spend a bit more for a good finish. Don't forget your card will often be the first thing your customer sees.
And has been said before - then block their e-mails :-)
Alan.