Multigigabit is mainly to address 802.11AC wave AP that requires more than 1G. (A link aggreation should ideally be a workaround here). There is roadmap for EX3400-MG perhaps in Q3'18 but not committed. This depends on business use cases/cost deployment as whole and couple other sales factors. For more details on Multigigabit roadmap for EX3400, you may reach out to your local Juniper accounts/sales representative.
For VC license, Refer to the following doc: