The problem we are trying to solve is that smart inventories in Tower do not include groups, and therefore we can't use group_vars defined at the inventories level. Since host_vars are imported we will use a host_var to trigger import of the wanted vars.

1- VM Properties

We're going to use the "Custom Attributes" feature of vSphere to inherit a host_vars. Each VM has a custom attribute dcl-custom-attr but with a diferent value for each of our test VMs:

2- Dynamic Inventory & Smart Inventory in Tower

We're using the VMware dynamic inventory with source variables:

We want to test importing vars for hosts sourced from a smart inventory so we define a smart inventory:

The VMs in the smart inventory show the custom attributes as host_vars:

3- Conditional include of vars

Now we can use the following playbook:

---
- name: Conditional vars inclusion based on host_vars 
  hosts: all
  gather_facts: no

  tasks:

    - name: Extract Custom Prop Value
      set_fact:
        vm_props: "{{ value | map(attribute='value')| list }}"

    - name: Include for first group
      include_vars:
        file: first.yml
      when: '"dcl-1" in vm_props'

    - name: Include for second group
      include_vars:
        file: second.yml
      when: '"dcl-2" in vm_props'

    - name: Display imported value
      debug: 
        msg: "{{ my_imported_value }}"

Depending on the value of the host_var we include a file or another, that defines the variable my_imported_value but with different values.
See: https://github.com/automaticdavid/set_vars

We define the associated Tower template:

Execution yields the inclusion of the var with the correct value for each VM:

We can use the same mechanism to include different vars for each VM.

This could be used in a role that would get included at the begining of each playbook, or with a Tower Workflow.