What should an architect use as a guideline to determine when to define another VLAN...

The guideline for defining another VLAN for wireless devices should be based on the employee roles, with a different VLAN for each role or department. This approach allows for better control and management of network resources. It also enhances security by segregating network traffic based on roles or departments. This is particularly useful in large organizations where different departments may have different network requirements and security levels12.

References:

Configuring VLANs - Aruba

TECH BRIEF SINGLE VLAN DESIGN FOR WIRELESS LAN - Aruba