HitFilm works fine with both VP12 and 13 installed as long as long as HitFIlm is installed last after VP 13 is installed. I would reboot after installing VP 13 before you install Hitfilm.
I did that too, removed both V13 & Hitfilm.. did a fresh installation.. with VP13 first followed by Hitfilm rebooted in-between both installations.. Did you by any chance have to remove your V12 as well before reinstalling Hitfilm? I'm wondering if there is a conflict between V12 & V13?