Please try out /etc/map mapping for this scenario. It allows for domain mapping so you can put entries which allow you to map exact domain. One more advantage is that /etc/map respects runmodes so you can pretty much create this for any environment.
Your prblem is that you want to reuse an already existing vanity URL. Vanity URLs can exist only once per server, it cannot point to both locations. Personally I wouldn't use it, but rather the map the resource path to a proper URI using resource mapping. That's much more convenient and scales much better (assume that you want to have "nice" URLs also for other pages). Others in this thread have already pointed out URLs where you can read how you can achieve that.