This is an important question, as the same thing happened back in E3 2006. FFXII was not even released in NA/EU but Squenix still announced FFXIII with a trailer that showed in-game graphics and even battle system gameplay. I visited various forums in the weeks after and there were a lot of people that were no longer hyped for FFXII, despite it launching later in the year.
They were saying things like:
-- "FFXIII looks amazing! I can't believe those graphics are real-time on PS3! It looks much better than FFXII and is also being made by the FFVII/FFVIII/FFX team! Man, forget FFXII. A PS3 and this game is now what I'm saving up for."
-- "FFXIII has the best battle system in the series. The animations are amazing and fluid and you can slow down time and make enemies hit the environments. I thought FFXII looked good but this blows it out of the water. I'm skipping FFXII and getting a PS3 for this game."
-- "I was gonna buy FFXII, but seeing as they're already making FFXIII and it looks like a playable FFVII Advent Children, I'll just buy a PS3 and wait for FFXIII to release."
As you can see, FFXIII took away some of FFXII's sales due to being announced before FFXII was released. Will the same happen with Versus XIII and FFXV?