A vulnerability while possible, is unlikely. The network has a difficulty barrier which prevents malicious attacks and limits the effects in such a situation. It is also fully decentralized, so there is no central server or body which can be exploited and lead to a crash.
Neither the combined honest hash power which sets a high difficulty for mining a block, nor the fact that blockchain is decentralized, protects against a vulnerability in the code. They both do increase the security against certain factors - the hash power against a 51% attack, and decentralization against a single entity seizing control of the network - but vulnerabilities or bugs in the code can and do happen despite all the eyes on it. Sometimes they are picked up and patched before they can be exploited, and in other cases they are exploited first and require emergency patches to be deployed.
The sudden inability to send or confirm transactions or the discovery of a critical vulnerability that would require a hard fork, which would divide the community
I don't think it would. The community - miners, nodes, developers, etc. - are all invested in to bitcoin. If bitcoin disappears or falls to zero, then they lose a significant amount of money, time, resources, etc. If there was suddenly some critical vulnerability discovered which allowed someone to DDOS the entire network and prevent all transactions, all honest users would accept a fork to fix it very quickly.