Could have sworn it GPed lows, atleast in arcade. Well, that explains why I keep getting hit by sweeps

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For Double Vapor Cannon, it visually has 360 degree guardpoint, but you're right. It does not have GP low (Which is misleading).
I'll make the necessary changes right now.
EDIT: In terms of guardpointing too late, it depends on when the move is active, when they recover, and when you started your move.
For example, if using far st.C or far st.D, pressing it off timing with the opponent's attacks will be fatal. You have to press it just as they're about to hit, but before the frames for the move become active. When the move becomes an active hitbox, the guardpoint frames are done (Which leads me to believe that you can only GP during startup of your move).
Also, you cannot guardpoint when the move becomes active if your hurtbox overlaps with it. You need atleast a frame to get the startups going. I don't know if it's in the first frame of the move that it starts to guardpoint, but you won't guardpoint any meaty moves after being knocked down.
You can also GP a move, but the time it takes to become an active hitbox could be slower. For example, Maxima uses st.D to guardpoint a hop CD from Mr. Karate. If he does it really late to the ground, he'll land much faster than my guardpoint becoming active. Even a guardpoint cancel might go right over him. You'd have to cancel immediately into a real fast EX special.
So because of these two instances, you can narrow down guardpoint active frames into the startup frames. However, if gp becomes active in the first frame of a moves animation (1st frame of start up), I don't know. Then it would be an actual parry with a HUGE buffer time.