This is what Drugs.com Drug Interaction checker says about the effects of Vicodin mixing with Valium
MANAGEMENT: During concomitant use of these drugs, patients should be monitored for potentially excessive or prolonged CNS and respiratory depression. Ambulatory patients should be counseled to avoid hazardous activities requiring mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.
It doesn't saying anything one drug negating the effects of the other.
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