An alias is just an extra IP address added to the same interface. It can be from the same subnet or from a different one. But anyone who can join to your LAN can access both subnets just by adding new IP to their PC. You cannot manage it and secure it. Also anyone can listen to your traffic.
A VLAN interface is an interface receiving 802.1q tagged frames from the host interface. When you're using VLANs, you're creating virtual LANs. These LANs are strictly separated, node from one VLAN cannot see communication in different VLANs.