It depends on how the VLAN tagging is applied. If the rule is that everything from a 2nd SSID has the VLAN applied, that's pretty secure. If the rule is that

It depends on how the VLAN tagging is applied. If the rule is that everything from a 2nd SSID has the VLAN applied, that's pretty secure. If the rule is that things with a particular MAC address have the VLAN applied, that depends on the device not spoofing another MAC address. And if your rule is that already tagged traffic is left as-is, then a device can tag its own traffic with another VLAN.