ISP -------- VR1 A ----- SRX/UDP FW -------- VR1 B return -------- VR2 (Vlans / Static routes ....etc) ------ EX4500
Yes that should be the solution :
For the internal gateways, you might consider creating a new virtual router routing-instance on the MX.
Place the interface leaving the SRX to the internal network into this routing-instance
Place the uplink from the EX to the MX into this routing-instance
Create all your internal routed vlan interfaces in this routing-instance
If we should add family inet firewall filters for input on VR1A and VR1B
but i do not know how to do it 
do you have any sample for this
BGP Needs to be on VR1 with upstream ISP
VR1 -- SRX
VR1 -- VR2 connections must be metric or OSPF
