Hi,
With VM2.6, it is possible to add reCaptch to "Ask a question" form, but "Registration Form" or "Guest Checkout Form" is not one of them.
Doing custom code changes won't be enough, because you need to put the codes to "designated" pages (to check if the reCaptcha has entered correctly or not) which requires some Core Hack to Virtuemart files. (it is not recommended to touch any core VM files)
You need a 3rd party extension for this purpose, it will save you hours. I know a Free Plugin does this job, it is called:
EasyCalcCheck
extensions.joomla.org/extensions/access-a-security/site-security/captcha/11964
It is capable to do it without any core hacks. You should use that free plugin.
Read this topic for more info:
www.flexiblewebdesign.com/forum-virtuemart-2-template-support/1986-captcha
This has been discussed before in this forum.
Regards,
Support