You are using the select filter wrongly with rather weird argument (I suspect a copy/paste error but I'm not sure). select will apply a test to each object in the list. I don't know any search test that can be applied to a hashmap (The closest I can think of is the python search method of an re -i.e. regexp- object which would not be appropriate anyway)

In your case, you are looking for a specific value of an attribute of your hashmap. This can be done with the selectattr filter which will apply a test to a given attribute of the objects in the list and return only the ones passing the test.

There is a different approach to your problem which is more compact IMO using the json_query filter

Below is an example playbook using both approaches leading to the same result.

---
- name: Sum size of FS
  hosts: localhost
  gather_facts: false

  vars:
    FS:
      - nom_FS: /appm/oracle/product
        nom_LV: lv_product
        size_FS: 5
        owner_FS: oracle
        group_FS: dba
        vg_name: vgapplis

      - nom_FS: /appm/oracle/product/12.1.0.2
        nom_LV: lv_12102
        size_FS: 15
        owner_FS: oracle
        group_FS: dba
        vg_name: vgapplis

      - nom_FS: /apps/oracle/logs
        nom_LV: lvlogs
        size_FS: 5
        owner_FS: oracle
        group_FS: dba
        vg_name: vglogs

  tasks:

    - name: Calculate with selectattr, map and sum
      debug:
        msg: "{{ FS | selectattr('vg_name', '==', 'vgapplis') | map(attribute='size_FS') | list | sum }}"

    - name: Calculate with json_query
      vars:
        sum_query: "[?vg_name=='vgapplis'].size_FS | sum(@)"
      debug:
        msg: "{{ FS | json_query(sum_query) }}"

And the result

PLAY [Sum size of FS] ****************************************************************

TASK [Calculate with selectattr, map and sum] ****************************************
ok: [localhost] => {
    "msg": "20"
}

TASK [Calculate with json_query] *****************************************************
ok: [localhost] => {
    "msg": "20"
}

PLAY RECAP ***************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   

Create a dictionary of usernames and their visibility. For example, given the below data for testing

  result:
    json:
      - {username: mike, visibility: private}
      - {username: alice, visibility: public}
      - {username: bob, visibility: top secret}

Declare the dictionary

  username_visibility: "{{ result.json|
                           items2dict(key_name='username',
                                      value_name='visibility') }}"

gives

  username_visibility:
    alice: public
    bob: top secret
    mike: private

Now, the testing is trivial

  username_visibility.mike == 'private'

shell> cat pb.yml
- hosts: localhost

  vars:

    result:
      json:
        - {username: mike, visibility: private}
        - {username: alice, visibility: public}
        - {username: bob, visibility: top secret}

    username_visibility: "{{ result.json|
                             items2dict(key_name='username',
                                        value_name='visibility') }}"

  tasks:

    - debug:
        var: username_visibility

    - assert:
        that: username_visibility.mike == 'private'

shell> ansible-playbook pb.yml

PLAY [localhost] *****************************************************************************

TASK [debug] *********************************************************************************
ok: [localhost] => 
  username_visibility:
    alice: public
    bob: top secret
    mike: private

TASK [assert] ********************************************************************************
ok: [localhost] => changed=false 
  msg: All assertions passed

PLAY RECAP ***********************************************************************************
localhost: ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

Is selectattr filter expecting all the dicts in the list to have the same keys?

More precisely, it is expecting all dicts in the list to have the attribute you are selecting on. If not all dict in the list have it, you will have to first filter out the items where it is not defined. This can be done with selectattr as well. (thanks @Randy for making this clearer since my initial answer).

In your situation, the json_query filter (which implements jmespath) can also do the job in sometimes a more compact manner. But it is not a core filter and requires to have the community.general collection installed.

Here are a few examples taken from your above requirements solved with both core filters and json_query solutions.

The playbook:

---
- name: "Filter data with core filters or json query"
  hosts: "localhost"
  gather_facts: false

  vars:
    # Your initial data on a single line for legibility
    test_var: [{"vm":"vm1","ip":"10.10.10.1"},{"vm":"vm2","ip":"10.10.10.2"},{"test_vm":"something","process_1":"X","process_2":"Y","process_3":"Z"},{"another_vm":"something_other"}]

  tasks:
    - name: Get objects having vm==vm1
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('vm', 'defined') | selectattr('vm', '==', 'vm1') | list }}
          With json_query: {{ test_var | json_query("[?vm=='vm1']") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

    - name: Get all objects having vm attribute
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('vm', 'defined') | list }}
          With json_query: {{ test_var | json_query("[?vm]") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

    - name: Get all objects having process_2 attribute
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('process_2', 'defined') | list }}
          With json_query: {{ test_var | json_query("[?process_2]") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

    - name: Get only a list of process_2 attributes
      vars:
        msg: |-
          With core filters: {{ test_var | selectattr('process_2', 'defined') | map(attribute='process_2') | list }}
          With json_query: {{ test_var | json_query("[].process_2") | list }}
      debug:
        msg: "{{ msg.split('\n') }}"

which gives:

PLAY [Filter data with core filters or json query] *********************************************************************

TASK [Get objects having vm==vm1] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: [{'vm': 'vm1', 'ip': '10.10.10.1'}]",
        "With json_query: [{'vm': 'vm1', 'ip': '10.10.10.1'}]"
    ]
}

TASK [Get all objects having vm attribute] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: [{'vm': 'vm1', 'ip': '10.10.10.1'}, {'vm': 'vm2', 'ip': '10.10.10.2'}]",
        "With json_query: [{'vm': 'vm1', 'ip': '10.10.10.1'}, {'vm': 'vm2', 'ip': '10.10.10.2'}]"
    ]
}

TASK [Get all objects having process_2 attribute] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: [{'test_vm': 'something', 'process_1': 'X', 'process_2': 'Y', 'process_3': 'Z'}]",
        "With json_query: [{'test_vm': 'something', 'process_1': 'X', 'process_2': 'Y', 'process_3': 'Z'}]"
    ]
}

TASK [Get only a list of process_2 attributes] *********************************************************************
ok: [localhost] => {
    "msg": [
        "With core filters: ['Y']",
        "With json_query: ['Y']"
    ]
}

PLAY RECAP *********************************************************************
localhost                  : ok=4    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0