Can I use E85 in my 1999 Plymouth Grand Voyager

Not a good idea. E10 is the max you should use.

It probably would not even run on E85 and E15 could damage the fuel system.

if its designated as flex fuel you can

That's why it's called flex fuel the computer switches over recognizes the smell of alcohol and puts on its dancing shoes and turns up the air-fuel mixture thus enabling it to run and it does run damn good but you get for mileage as it takes a lot more liquid to burn it comes with flat top high-compression pistons and instead of paper or rubber gaskets it has metal and copper in certain places if it is a flex fuel vehicle it will run on 85 idiots that said it won't or only 10 or 15 will damage the motor they put 15 to 20% alcohol in our gas already so that they can put more water in frackers