As long as the car is in good condition as you stated, I wouldn't let the miles worry you. It can be hard to find a quality Viper now-adays at the right price, so if this car seems like a good deal for the quality and price, you should get on it fast. One of the biggest factors I put into play besides mileage when I bought mine was how many previous owners and was it stock. For the best piece of mind, it's definitely a way better situation to know the car you are buying was bought and owned by one person and didn't mod it, so you know they most likely baby'd it and took real good care of it over the years.
Anyway, to get back on topic. Unless it's a Ferrari or some type of imported exotic where they require a lot of maintenance and can break down a lot(which a Viper doesn't), 28,000 miles isn't really high mileage at all. Just like 28,000 miles wouldn't be considered high on any 9 or 10 year old car. I almost bought a 2000 Steel Grey GTS with 28,000 miles in California about 3 years ago, but fortunately found my 96 B&W I'd been waiting for instead with 9,181 miles which is of course very low miles. I also paid a good amount for it, but being a 96 B&W GTS I got a great deal. It's funny how 5-10,000 miles can greatly increase the price of a car.
I'm actually about to hit 27,600 miles on my GTS. The engine isn't even broken in on the Viper until you hit at least 30-35,000 miles anyway.