I have MX 400 and MS 350. I have created VLANs in MX 400 until now. I think there are two ways to create a VLAN that needs to be connected to the Internet. 1: Create a VLAN on the MX 400. 2: Create a VLAN in MS 350 and route it to VLAN 1 (Default) of MX 400. Please advise which of 1: 2: is more advantageous in terms of security and operation. Thank you.
Not using VLAN 1 is pretty much the only suggestion since there is no VTP on Meraki so not to much benifit to staying inside standard VLAN range.
But any VLAN you want to reach other side of MX you will need to configure in MX. Any switch you want to have ability to use VLAN will need VLAN configured.
Layer3 switches you will also want to configure an interface so they can route packets between VLANS. Be sure not to reassign interface IPs when doing this.