It's WIF (Wallet Import Format), and AFAIK, vanitysearch doesn't have a converter command.

If you want to know how, it's not that complicated, you just need some tools for SHA256 computation (Hash online tool) and Base58 encoder/checker (Base58 online tool),
Then, follow these steps:

• 1. Get the private key hex,
  Sample: 4116A49CA05407397C159D9F8EFC32064E3165466173AA26D21DA11A169BF7FD
• 2. Insert 80 at the start of the prv key and 01 if you want the compressed prv key WIF (do not insert 01 if uncompressed).
  Sample Compressed: 804116A49CA05407397C159D9F8EFC32064E3165466173AA26D21DA11A169BF7FD01
  Sample Uncompressed: 804116A49CA05407397C159D9F8EFC32064E3165466173AA26D21DA11A169BF7FD
• 3. Get the SHA256 Hash of the string above:
  Sample Compressed: 47aa53130b00c52fed1cf223a04bce7a2d083f111ed5b3dcb6a2a23f06dc3c78
  Sample Uncompressed: 1a75a1d066dcc407ac94a8e07042bb3fcf97f0c41b9d23c2ec9b54fc75b8e2c4
• 4. Get the SHA256 Hash of the SHA256 Hash above:
  Sample Compressed: 78a9182fdc5389e180d08702ac1ec0707221c861b08a7c0e3da56465c01104f9
  Sample Uncompressed: 7b376ba10130e9ae74f447847fbc5ae1c076d42efc932e8dc30ca64401835c8f
• 5. Get the first four bytes of the Double SHA256 hash above (the first 8 characters):
  From Compressed: 78a9182f
  From Uncompressed: 7b376ba1
• 6. Then insert the result of step5 after the result of step2:
  Sample Compressed: 804116A49CA05407397C159D9F8EFC32064E3165466173AA26D21DA11A169BF7FD0178a9182f
  Sample Uncompressed: 804116A49CA05407397C159D9F8EFC32064E3165466173AA26D21DA11A169BF7FD7b376ba1
• 7. Encode to Base58 the strings above and you'll get the WIF format:
  Compressed WIF: KyQEYWYtkttfuaMGBqoBqtpkUJ4M8yHKVtQ3pBkXChBXbraGiS2z
  Uncompressed WIF: 5JJxBK2LRu5umY7rfFgTvfvsBGCBTGvYNx9Pqkfrtb2Un7BrosE