Verbal employment contracts are enforceable in in all 50 states.
When you have a written contract, any and all verbal contracts or amendments mean absolutely nothing. It's called the "four corners doctrine" and is a pretty universal judicial practice. Written contracts always, ALWAYS supersede any verbal agreements: with a written contract in hand, any mentions of oral discussions are irrelevant and will fall on deaf ears.
FWIW the Wikipedia definition, http://en.wikipedia.org/wiki/Four_corners_%28law%29, contrasts sharply with your use and doesn't really seem to speak at all to overwriting of terms by further contracts.
Yes, if A and B walk into court and say that the verbal contract exists, it is enforceable. But in the common case, A will deny that the contract existed (or disagree on the terms) AMD without any evidence, the contract is lost.
Now if there is evidence (both sides had taken specific actions obviously in accordance with the verbal contrac, that they otherwise would have no reason to do), ther might be a case. But really, without evidence, the abstract rule will not connecg to concrete instances. Get evidence for everything.
Its sort of like 'copyleft' except it sucks into Google anything you touch, and use at least once at work.
And always be dubious when the company lawyer says, "Well that is what it says but that isn't what is meant by that clause." Because in a court of law, what it says counts.