Vidura helped PAndava at every chance.
The wicked Duryodhana, beholding Bhimasena surpass (everybody) in strength and Arjuna highly accomplished in arms became pensive and sad. Then Karna, the offspring of the Sun, and Sakuni, the son of Suvala, endeavoured by various means to compass the death of the Pandavas. The Pandavas too counteracted all those contrivances one after another, and in obedience to the counsels of Vidura, never spoke of them afterwards. [source]
We can very well derive the then dynamics of the palace politics. The reason for keeping quiet can be many:
- Vidura may loose his post, had he openly condemn those techniques; Then no secret saviour of PAndava-s
- Even had he condemned openly, that would have brought infamy to the whole Kuru race (including PAndava), because such things spread quickly among the citizens, which would reduce the trust towards their rulers
Vidura was aware of Dhritarashtra sanctioning the wicked plan of burning them alive; Being an obedient minister, he couldn't openly oppose his own elder brother
Then the son of Suvala (Sakuni), king Duryodhana, Duhsasana and Kama, in consultation with one another, formed an evil conspiracy. With the sanction of Dhritarashtra, the king of the Kurus, they resolved to burn to death Kunti and her (five) sons. [source]
Vidura was not a real threat to Duryodhana & co.. This is how the plan of "Varnavrata" itself got materialise
Duryodhana answered, '... Kshattri (Vidura) is dependent on us for his means of life, though he is secretly with the foe. It he sides the Pandavas, he alone can do us no injury, Therefore, exile thou the Pandavas to Varanavata (place of house of lac) without any fear. [source]
Nevertheless, Vidura didn't remain silent towards Kunti; He informed them about the treachery & also helped by letting Kunti & sons escape, beforehand. He just kept mum towards DhritarAshtra & sons, as there was no use of preaching them when they were devoid of morality already.
Here how the story of LAkshyagruha @ Varnavrata happened in very brief:
Then the sinless Vidura, of soul enlightened by true knowledge, and devoted to the good of the Pandavas, came to the conclusion that Kunti with her children should fly away from her foes. And providing for that purpose a boat strong enough to withstand both wind and wave, he addressed Kunti and said, 'This Dhritarashtra hath been born for destroying the fame and offspring of the (Kuru) race. Of wicked soul, he is about to cast off eternal virtue. O blessed one, I have kept ready on the stream a boat capable of withstanding both wind and wave. Escape by it with thy children from the net that death hath spread around you.'
Kunti was deeply grieved, and with her children, O bull of Bharata's race, stepped into the boat and went over the Ganges. Then leaving the boat according to the advice of Vidura, the Pandavas took with them the wealth that had been given to them (while at Varanavata) by their enemies and safely entered the deep woods. In the house of lac, however, that had been prepared for the destruction of the Pandavas, an innocent Nishada woman who had come there for some purpose, was, with her children burnt to death. And that worst of Mlechchhas, the wretched Purochana (who was the architect employed in building the house of lac) was also burnt in the conflagration. And thus were the sons of Dhirtarashtra with their counsellors deceived in their expectations. And thus also were the illustrious Pandavas, by the advice of Vidura, saved with their mother.
As you see, the help from Vidura here was quite explicit. It was merely a lucky coincidence for him that Duryodhana etc. got deceived due to circumstances. Else upon inquiry, Duryodhana might have found out some insider's help in escaping of PAndava-s.
The language used by Vidura to Yudhishtira can be found in this chapter. Due to it's length, I am just putting its meaning:
Kunti approached Yudhishthira and said, 'The words that Kshattri said unto thee in the midst of many people so indistinctly as if he did not say anything, and thy reply also to him in similar words and voice, we have not understood. ... Yudhishthira replied, 'The virtuous Vidura said unto me that we should know that the mansion (for our accommodation at Varanavata) hath been built of inflammable materials. He said unto me, 'The path of escape too shall not be unknown to thee,'--and further,--'Those that can control their senses can acquire the sovereignty of the whole world.'