Welcome to the forums @Rod, as others have already mentioned the MX is your best option for setting up your VLANs and DHCP. My reasons for this are below but these are based on a small network if you are building a large L3 network I would suggest doing this on your core switch
1. You are less likely to reboot an MX unit than a switch which means DHCP leases aren't going to be affected.
2. If you add multiple switches you might not always have every VLAN going everywhere and having DHCP on a switch in this instance could get messy.
3. Its easy to remember, the switches to the switching and the MX does your DHCP, VLANS and routing.
As I said above though if your network is going to be a resonable sized network I would setup everything on a core switch.
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