With a good CoO and the fact that he was in his "room", he should have been able to deal with that elbow punch a lil better.
The elbow strike came after a heart squeeze.
There was nothing he could do about it. Judging from how beat up Law already looked when it cut to their fight I'd say it is reasonable to assume that Vergo has been exploring the heart thing since the beginning.
Law has beastly reaction speed, but he is in way too big a disadvantage in this fight.