Per my comment:
"It seems like you're asking two different questions combined into one. One part seems to be about whether the DM should tell the player that the enemy is resistant/vulnerable or not. The other part seems to be whether the damage dice should be halved/doubled, or the damage itself should be halved/doubled. (This part is addressed by the rules; have you read them?) However, it's not clear why you're assuming that not telling the player = halve/double the actual damage, and that telling the player = halve/double the number of damage dice. Could you clarify your reasoning there?"