You don't float living, frantic, poison-spike-covered little rabbit monsters in pitcher plants. A fly that's all weighted down in plant juice, sure, but I have a hard time seeing a victreebell never getting pricked by it spiny prey it swallows whole.
Presumably it has tough insides to keep its own acid from eating at itself, so I would assume that it's tough enough that the Victreebel doesn't care about small pokes, especially from another poison type.