I'd just wait until you pull another one and merge him honestly. VF is on a good number of units these days, and Greil isn't the best green armor. Legendary heroes are hard to pull and therefore are more of a sacrifice to use as fodder.
That being said, if you wnana build Greil, go for it. I'd at least wait till next month and inherit special fighter to him along with DC from V!Ike if you go through with foddering him off