Clarify raw vLLM video sampling requests

.agents/skills/cookbooks/cosmos3/reasoner/README.md

@@ -129,6 +129,33 @@ response = client.chat.completions.create(
 print(response.choices[0].message.content)
 ```
 
+For video requests, frame-sampling controls are multimodal processor options.
+Assuming `messages` contains the video content for your request, pass those
+controls via `extra_body` when using the OpenAI client:
+
+```python
+response = client.chat.completions.create(
+    model=client.models.list().data[0].id,
+    messages=messages,
+    extra_body={"mm_processor_kwargs": {"fps": 4, "do_sample_frames": True}},
+)
+```
+
+For direct `/v1/chat/completions` HTTP requests, put `mm_processor_kwargs`
+inside the JSON body as shown below. Do not send `fps` or
+`do_sample_frames` as standalone top-level chat-completion fields.
+
+```python
+payload = {
+    "model": "Cosmos3-Super",
+    "messages": messages,
+    "mm_processor_kwargs": {"fps": 4, "do_sample_frames": True},
+}
+```
+
+This matters for motion-dependent tasks such as physical plausibility analysis,
+where frame sampling determines which temporal events are visible to the Reasoner.
+
 ### Notebook walkthrough
 
 [`run_with_vllm.ipynb`](./run_with_vllm.ipynb) uses the **Cosmos3-Super** launch