Grand Viper is unsafe even if you successfully hit your opponent with it. That is if your opponent have Burst. During GV hit state before going into VV animation your opponent can actually Burst and land safely on the ground, leaving you in the air whiffing your GV. After you land you'll get punish for it. IIRC, there's no knockdown on the burst, so you'll go straight right through your opponent thus whiffing your GV. Correct me if I'm wrong though, that's what I usually see on match videos. Also I've already done this a few times, don't know if it's just a fluke or if it's really do-able.
So yeah, GV is unsafe even on hit as long as your opponent have Burst.