Hi,
I dont know how that plugin works but it is normal if it doesnt work in TAB section by default, because tabulated form comes with our Template, and your plugin was designed for default settings, i cant help you to enable the captcha for tabulated ask question form, because it is a customization and our free support doesnt cover any kind of custumization, but you might want to disable the tabulated ask question form though, so the form will be reached only after clicking the link. To disable the ask a question form in footer TABS section; open this PHP file.
ROOT/templates/<yourjolatemplate>/html/com_virtuemart/producdetails/default.php
(Or flexv2.php, depends on which one you are using)
And look for the the ask question form's HTML and PHP lines, simply eliminate them.
Cheers.