docs: update vllm mermaid diagram (#4320)

Signed-off-by: PeaBrane <yanrpei@gmail.com>

docs/backends/vllm/README.md

@@ -86,18 +86,32 @@ This includes the specific commit [vllm-project/vllm#19790](https://github.com/v
 
 This figure shows an overview of the major components to deploy:
 
-```
-+------+      +-----------+      +------------------+             +---------------+
-| HTTP |----->| dynamo    |----->|   vLLM Worker    |------------>|  vLLM Prefill |
-|      |<-----| ingress   |<-----|                  |<------------|    Worker     |
-+------+      +-----------+      +------------------+             +---------------+
-                  |    ^                  |
-       query best |    | return           | publish kv events
-           worker |    | worker_id        v
-                  |    |         +------------------+
-                  |    +---------|     kv-router    |
-                  +------------->|                  |
-                                 +------------------+
+```mermaid
+%%{init: {'theme':'base', 'themeVariables': { 'fontSize':'10px', 'primaryColor':'#2e8b57', 'primaryTextColor':'#fff', 'primaryBorderColor':'#333', 'lineColor':'#81b1db', 'secondaryColor':'#b35900', 'tertiaryColor':'#808080', 'edgeLabelBackground':'transparent'}}}%%
+graph TD
+    %% Node Definitions with custom shapes
+    HTTP[HTTP]
+    ROUTER[Router]
+    PREFILL[vLLM Prefill Worker]
+    DECODE[vLLM Decode Worker]
+
+    %% Class Definitions for color
+    classDef worker_style fill:#2e8b57,stroke:#333,stroke-width:2px,color:#fff;
+    classDef router_style fill:#b35900,stroke:#333,stroke-width:2px,color:#fff;
+
+    %% Applying classes to nodes
+    class PREFILL,DECODE worker_style
+    class ROUTER router_style
+
+    %% Request/Response flow
+    HTTP <--> |"request/response"| ROUTER
+    ROUTER --> |"1. send to prefill"| PREFILL
+    PREFILL --> |"2. return NIXL metadata"| ROUTER
+    ROUTER --> |"3. send with metadata"| DECODE
+    DECODE --> |"4. stream response"| ROUTER
+
+    %% KV Events publishing
+    PREFILL -.-> |"publish kv events"| ROUTER
 ```
 
 Note: The above architecture illustrates all the components. The final components that get spawned depend upon the chosen deployment pattern.