Answers in RED:
If I am going to implement VSAN with SRM, the only replication that I can use is vsphere replication since SRA will work if I have an external storage for another external storage replication? - correct, if you are using VSAN you have to use VM level replication as there is no SRA for VSAN
So if they need a near 0 RPO, VMs they need to protect should be in the external storage instead on the vSAN datastore and SRA will be used? - If you need less than 5 min RPO you would want to look at either array-based replication that supports a lower RPO or a solution like RecoverPoint For VMs from EMC which provides a lower RPO. Note that RP4VM doesn't integrate with SRM.
Do I need SRA if my placeholder datastore is an EMC storage? - You need an SRA if you are using array-based replication. If you are using vSphere Replication, regardless of where the placeholder datastore is located an SRA is not required/needed/used.
Is there any writeups or procedures where I can use the vSAN datastore as place holder datastore? I beleiev in one of my previous posts that it is doable, just create a folder or a part to be the place holder datastore, - See your previous post and the responses for details