The equivalent of Cisco VSS in Juniper is called MC-LAG. Basically, 2 different switches appearing as one to external devices with a dual, separate control plane. Generally, each switch must be configured on their own, although Juniper has some enhancements to make this type of config easier - I posted something on another thread about this. Both VSS and MC-LAG are limited to 2 switches max for the core nodes.
Cisco "stacking" and Juniper "virtual-chassis" are basically the same concept but in general allow for more than 2 switches in the 'stack' and have a single control plane, potentially with some redundancy built in. For Juniper, this would be master/back-up REs, and things like GRES, NSB, NSR, etc.
Juniper EX9200 is one EX model that does NOT support VC configuration. EX9200 in terms of node level redundancy and failover can be accomplished by using MC-LAG (search for this) or Junos Fusion for Enterprise (JFE) or now IP Fabric using EVPN/VXLAN (or EVPN/MPLS - EX9200 supports both).
I might suggest whomever you purchased these EX9200s from, you sit down and discuss design options and pros and cons of each approach.
Good luck!