Hi, in essence it is possible to put a captcha in, but it requires pretty extensive customizations to the virtuemart core code.
Most of the available captcha solutions for VM do only work half or not at all.
I have done some quite extensive work on it, the main problem is that virtuemart its own form validation is to say the least a bit buggy. Adding a captcha to that will only make it worse.
Recaptcha will probably not work for you because it uses javascript that does not play well with other javascript libraries.
It took me several weeks to get it to work the way I wanted; adding recaptcha and ajax form validation to virtuemart; and to be honest, it is still not completely done :oops:
I wouldn't recommend one, because I know you'll run into a lot of trouble. To make it work in all browsers is another painful experience.