I’ve been working on something related recently and noticed there are a number of settings that if changed might allow root login.
Search your machine for “tightvnc.conf” You should find one in /etc/ and a number of others in the various User directories. Read the comments, what you want may require a complex combination of settings (eg PAM on or off?) and any configurations in the User directories will over-ride what is in the system-wide file (/etc/).
You may find it’s not worth the trouble to try to login with root, just login with a normal User and su or sudo.