First of all, EVERY COIN is vulnerable to a 51% attack, just like your password and cryptogrpahy are vulnerable to a brute force attack. There is nothing that can prevent one. However, the costs of performing a 51% attack are extremely high and it is difficult to pull off. The community will react and come up with a solution, which might just be get more people to mine thus reducing the attackers percent of the hash rate. Also, a 51% attack is not the end of Bitcoin. All the attacker can do is double spend his own coins, attempt to prevent people from spending, and attempt to prevent miners from mining.

Even with 51% of the network, that doesn't mean the attacker suddenly has absolute power and full control. That just means that he has a higher probablility of finidng more blocks and could potentially create a longer valid fork of the block chain. The other 49% of the miners would still be finding about half of the blocks. In order to actually gain control of mining and adversely affect Bitcoin, the attacker would probably need somewhere upwards of 80 - 90% of the hash rate, which is much more expensive than just 51%.