Maybe the vice principal can be girlfriend number 100 (because she is so prone to french kiss guys and that would be too much for this manga also kusuri could give her the makes you young drug she took to make her prettier)
BUT then again, she loves kissing young boys and she would probably be unhappy only kissing Rentarou like she could always kiss the girlfriends but she is always kissing boys and has never be show kissing a girl before. They could always argue that she kissed a lot of boys because she had a hole in her heart that she couldn't fill and that hole was filled when she fell in love with Rentarou and kissed him but I would not believe that.
Yeah can't rule out any possibilities with manga and it's mangaka. I said this before but an alternative name for it could be "Rentarou's Bizarre Love Life" with the optional addition "with his 100 Girlfriends".
Because of the anime I have been rereading some of the first chapters and yeah this manga has always been weird
Has Rentarou and the Vice Principal ever had eye contact with her sun glasses on? I'm kinda interested if Rentarou will have a girlfriend who is not conventionally attractive. Like hate to say it but ugly maybe? Or maybe a girlfriend who is disabled like for example she needs to be on a wheelchair since she can't use her legs? Like 100 girls and they each have to different with 2 special traits to them.