1) Your VM's will all continue to work if vCenter crashes, however you will lose the most of the functionality vCenter provides - so no live migrations, however HA will still work. This is because vCenter is required to configure HA, but it sets it up so that it is not required for it to function. In practice we've found that removing vCenter primarily only impacts your ability to make changes, but things run smoothly without it.
2) It does not have to be on the same VLAN, it can be anywhere on your network that can communicate with the ESX(i) hosts.
As an aside when we first setup our cluster we were told running vCenter on a VM inside of the cluster it was managing worked fine - we were skeptical but tried it. It's worked fantastic for the last nearly 4 years with no problems. It seems deeply wrong somehow - but it works great.
(Edited answer to #2 to reflect correction in comments)