So forgive me if this isn't entirely likely or anything but, you have to consider how fast Ivy can swing that thing safely (as in, how fast she can pull it back without it lodging in her own brain).
Judging from how far apart the bladed bits of the whip are, it seems -POSSIBLE- that Viper could actually grab it and either yank it from Ivy or give it a jolt of electricity. I mean yeah, that's a pretty big risk and all but Cloud can disarm Crono and people in real life can do crazy stuff like catch arrows if they're trained properly. I assume Viper's spy training would at least give her not only heightened combat abilities but she'd probably also need pretty good reaction speed if push came to shove.
Plus Ivy's got those weird heels on; if Viper gives her a good tug, it's possible that, with Ivy's poor footwear choice and with the surprise of having her weapon tugged on quite hard that she'd lose her balance. Granted Ivy could also pull it back and tear at Viper's hands but she's still vulnerable when she's not swinging. Viper's a spy; she's fast and deadly; if she can get in, Ivy's as good as dead.
And I personally (please say so if you feel differently) don't feel that catching the weapon would be entirely out of the realm of possibility; look at a picture of Ivy with the whip sword out (I'd post one but be banned for pornography probably, thanks Namco), there are huge gaps between the blades that are what I assume to be either metal or rope which could easily be grabbed. Plus, as I actually look at the image, it would appear that the blades have a flat side where it would slot together if it were going to reform into a sword (as it does in the game, not in TDB). That would possibly mean a "safe" area for Viper even if Ivy tugged it back.
Just my two cents...