I heard this rumour that they people who make sesame đường phố, street turned, hoặc tried to turn, the cookie monster into the Vege Monster. True hoặc false?

yes it is true.

NO! they never tried to do that because they know that if they did they'd have alot of angry fans.
this is a website where bạn can read about cookie monster promoting healthy eating but still loving cookies. link

yes

I'm not exactly sure

Yep, that would be weird his letter wouldn't even be C anymore!!