The provided string appears to be a base64 encoded VMess protocol configuration, which is used for secure and encrypted communication over the internet.

Here's how you can decode it:

1. **Extracting the vmess:// URL**:

- The string starts with `vmess://`, which indicates that this is a base64-encoded VMess protocol configuration.

2. **Base64 Decoding**:

- To decode this, we need to remove the `vmess://` prefix and then decode it using a base64 decoder.

3. **Decoded JSON Configuration**:

- After decoding the string, we should get a JSON-formatted string that represents the configuration settings for the VMess protocol.

Here's an example of how you might decode this in Python:

```python

import base64

# Remove the 'vmess://' prefix

encoded_config = "eyJ2IjogIjIiLCAicHMiOiAiXHU1ZTdmXHU0ZTFjXHU3NzAxXHU1ZTdmXHU1ZGRlXHU1ZTAyIFx1NzlmYlx1NTJhOCIsICJhZGQiOiAiMTIwLjIzMi4xNTMuNjMiLCAicG9ydCI6ICI0MDU2NSIsICJ0eXBlIjogIm5vbmUiLCAiaWQiOiAiNDE4MDQ4YWYtYTI5My00Yjk5LTliMGMtOThjYTM1ODBkZDI0IiwgImFpZCI6ICI2NCIsICJuZXQiOiAidGNwIiwgInBhdGgiOiAiLyIsICJob3N0IjogIiIsICJ0bHMiOiAiIn0="

# Remove the 'vmess://' prefix

encoded_config = encoded_config.replace("vmess://", "")

# Decode base64 string

decoded_bytes = base64.b64decode(encoded_config)

# Convert decoded bytes to a JSON-formatted string

import json

config_json = json.loads(decoded_bytes.decode('utf-8'))

print(config_json)

```

This Python script will output the decoded JSON configuration for you.