@ANarcis You haven't answered @Ben but I'm going to assume that what he said is indeed what you're trying to do. The question whether you need a route on the MX pointing towards VLANs that are present on the Cisco router downstream depends on what you're trying to do and how the router is configured.
If addresses of those downstream VLANs need to be reachable from the outside, i.e. connections initiated from the outside (whether that be through port forwarding, 1:1 NAT, AutoVPN, ClientVPN, ...) then yes, you definitely need a route on the MX.
If you just need those VLANs to have connectivity to the internet then you could just turn on NAT-masquerading on the router. The disadvantage there is that you no longer have insight into the original source addresses of the packets as those will be overwritten by the router's outer-IP by NAT. I'm going to guess that that is not what you want. So if you want to do filtering on the MX you'll likely want the MX to know about the downstream VLANs. so you would not turn on NAT in the router and you would indeed need route(s) on the MX pointing towards the Cisco router.
I hope that clears up things.
