Yes, BMSs can communicate with ECSs in the same VPC.
Your VPC may consist of multiple network segments. If the BMSs and ECSs are in the same segment, they communicate with each other through the Layer 2 network. If they are in different segments, they communicate with each other through the Layer 3 network.
BMSs and ECSs can be placed in the same subnet. To enable communication between the BMSs and ECSs, you must configure security group rules.