Hi snaveen 
I think the JSON body of our request will look something like the following. Note that I removed the * from SELECT *. We recommend against using SELECT * as much as possible in LiQL queries (particularly for the Messages collection because a Message object is quite large) and instead be as specific as possible in the SELECT statement. While a query with * might technically return data, * is not officially supported in the SELECT statement.
Also, notice that I added both a LIMIT for both queries and I added a SORT option for the query to the Messages collection (also a best practice). See the Best Practices section of Using LiQL for more performance tips.
[
  {
    "floated_messages":{
      "fields":[
        "id",
        "href",
        "message",
        "scope",
        "user"
      ],
      "constraints":[
        {
          "message.board.id":"VxRail"
        },
        {
          "scope":{
            "matches":"global"
          }
        }
      ],
      "limit":5
    }
  },
  {
    "messages":{
      "fields":[
        "id",
        "view_href",
        "href"
      ],
      "constraints":[
        {
          "board.id":"VxRail"
        },
        {
          "id":{
            "matches":"8122626"
          }
        }
      ],
      "sorts":[
        "post_time desc"
      ],
      "limit":5
    }
  }
]
 
To build the JSON body for the request, I copied the format from the  Request body format for JSON-based queries section of the Community API v2 request guide and basically filled in the <field>, <collection>, and <sort> placeholders with the related fields, constraints, and sorts in the LIQL queries you posted.
A cURL example would look something like this using Session Key authentication. If you're using OAuth, then you'd use the client-id and Authorization headers (described in the Headers section of the guide I mentioned above) instead of the li-api-session-key header.
curl -L -X POST 'https://[COMMUNITY DOMAIN]/api/2.0/search' \
-H 'li-api-session-key: [SESSION KEY]' \
-H 'Content-Type: application/json' \
--data-raw '[
  {
    "floated_messages":{
      "fields":[
        "id",
        "href",
        "message",
        "scope",
        "user"
      ],
      "constraints":[
        {
          "message.board.id":"VxRail"
        },
        {
          "scope":{
            "matches":"global"
          }
        }
      ],
      "limit":5
    }
  },
  {
    "messages":{
      "fields":[
        "id",
        "view_href",
        "href"
      ],
      "constraints":[
        {
          "board.id":"VxRail"
        },
        {
          "id":{
            "matches":"8122626"
          }
        }
      ],
      "sorts":[
        "post_time desc"
      ],
      "limit":5
    }
  }
]'
Please let me know if this helps.