Researchers at the University of North Carolina-Chapel Hill found that diet soda drinkers who ate a so-called "prudent" diet, rich in fruit, fish, vegetables, whole grains, nuts and milk, were significantly less likely to develop metabolic syndrome over 20 years than those who ate a "Western diet" heavy in fried foods, meats and sugars.
The oral bacteria which cause heart disease require sugar and acid to function.
Diet soda doesn't contain the sugar, but is still acidic and compromises the teeth and mouth tissues.   If you also eat a diet with excess carbs, you will still be providing a nice environment for pathogens.