Organisms using thermal energy as their sole energy source
I had to re-read your last sentence a few times to make sure I understood it correctly, but I think that now I do, and I can answer your question.

What you're talking about are thermophiles. They're small organisms that love hot conditions - up to nearly 250 degrees Fahrenheit. They can be found places with a lot of hot water, such as hot springs - and, yes, ocean vents. Many use sulfur as a fuel during chemosynthesis, oxidizing it to make sulfuric acid. They take the energy from the heat source to do this.

Not all thermophiles use chemosynthesis. Those in hot springs may get a lot of sunlight, and they can use photosynthesis. However, those in the deep sea cannot undergo photosynthesis, and are forced to use the energy from the vents to undergo chemosynthesis. In fact, entire ecosystems can be built off them!

Thermophiles may even have been some of the first organisms on Earth. Around the time that life is thought to have first formed, Earth wasn't a very hospitable place. As Ashwini Kumar Lal writes of the first forms of life,


  Many of them are evolutionary relics called “archaea”, believed to be among the first homesteaders on the Earth 3.8 billion years ago. They are presumably
  the first version of life on our planet when its atmosphere was devoid of oxygen, and comprised largely of ammonia, methane, water vapour and carbon dioxide. They are microorganisms similar to bacteria in size and simplicity of structure, but continue as an ancient group intermediate between bacteria and eukaryotes. Heat – loving microbes, or “thermophiles”, are among the best studied of the extremophiles that can reproduce or grow readily in temperatures exceeding 45º C, and some of them, referred to as “hyperthermophiles”, are capable of thriving even thrive in temperatures as high as 110º C – more than boiling water temperature.


While we do not know (and may never know) the truth of this hypothesis, it is certainly plausible, given the locations where some extremophiles have been found.



References:

1: Microbial Life in Extremely Hot Environments

2: http://en.academic.ru/dic.nsf/enwiki/18641 (link no longer works, for some reason)

3: Origin of Life

Note: While I list several Wikipedia articles throughout the answer, I did not use them for the information in this answer. I think they're a good starting point to lead you to other sources, though.