entityDump.txt

...
    {
      "name": "Sample:SomeBlock",
      "persisted": true,
      "alwaysRelevant": false,
      "Sample:ThrowingBlock": {
        "x": 0.0,
        "y": 20.0,
        "z": 0.0
      }
    },
...

SomeBlock.block

{
    "basedOn": "core:rock",
    "displayName": "Hello Block",
    "penetrable": true,
    "entity": {
        "prefab": "Sample:SomeBlock"
    }
}

SomeBlock.prefab

{
  "ThrowingBlock": {
    "x": 0.0,
    "y": 20.0,
    "z": 0.0
  }
}

ThrowingBlockComponent.java

package org.terasology;

import org.terasology.entitySystem.Component;
//import org.terasology.math.geom.Vector3f;

public class ThrowingBlockComponent implements Component {
    //  public Vector3f velocity;
    public float x;
    public float y;
    public float z;
}

Working Vector3f version:

{
  "ThrowingBlock": {
    "velocity": [0.0, 20.0, 0.0]
  }
}

entity dump:

    {
      "name": "Sample:SomeBlock",
      "persisted": true,
      "alwaysRelevant": false,
      "Sample:ThrowingBlock": {
        "velocity": [
          0.0,
          20.0,
          0.0
        ]
      }
    },