This trip is for a pretty frum Chassidish couple. Don't think the guy would enjoy Paris that much. Vienna seems a bit more the type. Even just in terms of being comfortable walking down the street.
Just came back from my second trip to Vienna this December . Stayed at the Hilton which is great location . Can take the CAT train and walk 200 meters to entrance.
If you have Hilton Gold status the Lounge there is wonderful and breakfast snacks etc morning afternoon and night .
Dont know what religious restrictions would be , but anything that you could enjoy being on the 11th floor with beautiful views of the City is worth it if you can do .
Take the hop off and hop on Bus for 2 day excursion till you get to know the City
including the Palace outside the City . Kosher restaurants I cant help but Vienna is a beautiful City, with no run down sections in ringstrasse .
Plus the PH is opening in 2013 in the center of town
I will be going back for third trip next Dec