Before I jump into an installation, please, do I need vCenter server for HighAvailability feature or just 2 vSphere ESX would do it ?
How is the heartbeat done? Through some heartbeat storage as it is with XenServer?
Yes, you need a vCenter server for HA.
See the following link, it may be old but same basic premise
There are a number of requirements that you will have to meet to make VMHA work. Those requirements are:
1) VMware Infrastructure Suite Standard or Enterprise (no you cannot do it with the free ESXi nor can you do it with the VMware Foundations Suite).
2) At least 2 ESX host systems.
3) A shared SAN or NAS between the ESX Servers where the virtual machines will be stored.
Keep in mind that with VMHA the virtual disks for the VMs covered by VMHA never move. What happens when a host system fails is that the ownership of those virtual machines is transferred from the failed host to the new host. CPU compatibility between the hosts. The easiest way to test this is to attempt a VMotion of a VM from one server to another and see what happens.