Although, Asus has not made Vivobook S451, it certainly exists in the wild. It was spotted by prohardver, who has even posted several pictures of it. From a glance, it looks exactly like a 13 inch Apple Macbook Pro except for the fact that S451 has a 14 inch display.
Asus makes 11.6 inch and 15.6 inch, S551 variants as well.
Asus S451 specs include 15.6 inch display, Intel Core i5 4200U haswell processor, Nvidia GeForce GT 740M GPU, 6GB RAM, three USB ports (two of which are USB 3.0), HDMI, and Ethernet.
Out of its 6 GB RAM (4GB RAM is soldered) and 2GB RAM stick is removable. Price and release date will be known at IFA 2013.