When is a new block added to the bitcoin blockchain and is it vulnerable to modifications till then?
a block is assembled by the decision of a miner. If he wants to, he cannot include transactions at all. If he includes tx, then he uses tx out of his mempool. tx are normally verified before entering into a block. If the miner decides to tamper with a tx (change its contents), then when sending the block to the network, other nodes will reject this block, cause it contains an invalid tx.


  Until a transaction is added to a block ... how are the transactions tamperproof?


tx be tamperproof doesn't make sense, while the block is created. It is at the risk of the miner! If he wants to win the block reward, he better transmits correct blocks into the network. 
The whole idea of bitcoin is, to better play well and get incentivized for it. If you try to play against the network, you have enormous costs to carry.