The Manhattan D.A. prosecuting Harvey Weinstein says he's got something up his sleeve ... doubling down the criminal case will move forward despite some major setbacks.

Cyrus Vance Jr. told our producer at an event Thursday night in NYC ... he's confident the remaining counts against Weinstein will stick ... this after Vance agreed to dismiss one of the sexual assault counts after learning lead detective, Nicholas DiGaudio ﻿, allegedly engaged in witness tampering .

Check out the audio ... Vance won't take a stand with our producer on whether or not the detective's actions were criminal. Our producer persisted and said the detective was involved in all of the counts against Weinstein, so it stands to reason the entire case may be tainted.