I am unable to replicate this issue. Are you sure it's related to vBForms? Have you tried disabling the mod and seeing if the error still appears when replying to the thread?
I'm unable to replicate this issue even when ticking that checkbox in a thread created with vBForms.
Are you able to reproduce it 100% of the time on your site? Also, can you confirm that it does not happen if you tick the box in a thread not created with vBForms (or if you disable vBForms)?
If the answer to both those questions is yes, could you please PM me with an FTP account and an AdminCP account? If so, I'll get it solved for you ASAP