This is implemented in 4.1.x.
Some history: In 4.0.x we avoided AutoValidator because when 4.0.x was initially in development (during XF 1.0), XenForo's AutoValidator function was very basic and would not have worked in VaultWiki. AutoValidator was vastly improved in XF 1.1 and 1.2, but still would have required some semi-major changes to VW 4.0.x in order to implement it. Due to other improvements, these changes are no longer necessary in 4.1.x; just some minor tweaks (such as adding the class to forms). Since the minimum required XenForo version has been XF 1.2 for quite some time, the issues in earlier XenForo versions are no longer a problem.