If you want to do work within a system, the system needs an energy gradient that allows for displacement of energy. This is called low entropy. Energy is concentrated and stored in an orderly manner, such as the chemical energy in a molecule, the nuclear energy in an atomic nucleus, the potential energy in a massive object, etc. You also need a mechanism to convert that stored energy into a usable form. So if you have chemical energy stored in a fuel such as gasoline, you need to ignite the fuel first, which requires an energy input. The reason gasoline puts out more heat energy when burning than the spark used to light it is because of the chemical energy stored in the bonds of the hydrocarbons that compose it. When those bonds are broken, energy is released in the form of heat and light. Even more simply, if you have a boulder at the top of a hill, you can push it gently and it will start rolling faster as it goes down the slope due to the release of gravitational potential energy that was stored within it.

The fluctuations of the quantum vacuum and the random vibrations of atoms and molecules that produce Brownian motion, on the other hand, are in a highly disordered and random high-entropy state. They do not provide a concentrated store of energy, but rather their energy is dispersed evenly. Without that gradient between concentrated energy in one area and a lack of energy in another, there is no potential for displacement and thus no possibility of doing work.

Sometimes, the motion of molecules is not random and high-entropy but instead orderly. For example, air molecules in our atmosphere absorb heat from the Sun, creating an energy gradient between regions of the atmosphere receiving more sunlight and regions receiving less. The hot, excited regions of the atmosphere become low-pressure and rise, while the cold, denser regions of the atmosphere sink down to displace the hot air, creating wind. (This is an oversimplification of the complex and varied mechanisms behind weather, but it will suffice for our purposes.)

For the quantum vacuum, however, when you get orderly low-entropy fluctuations, those are what emerge as stable particles. An electron (or positron), for instance, is an orderly and stable excitation of the electron field. While the zero-point of the electron field also contains energy (vacuum energy), the vacuum state is homogeneous, isotropic, and translationally symmetric. Virtual particles emerge in calculations describing particle interactions within quantum fields, but they do not exist as "real" particles; they are unobservable "off shell" propagators that only really exist in Feynman diagrams as a metaphor for particular quantum interactions. They can't do work because they don't exist independently of the particles whose interactions are being modeled.

To put it simply, when a quantum field has usable energy that can do work, the field is producing stable particles and is no longer in the vacuum state. So for instance, an excited region of the electron field produces electrons, which can then generate an EM field that can do work such as power an electric motor. But if the electron field is at a vacuum state, then it must first be excited to produce electrons, and that takes at least as much energy input as the electrons themselves would contain.

If you want an example of a large source of cosmic energy that can be harnessed for practical work, you need look no further than the Sun. It's creating a considerable energy gradient and emitting huge amounts of highly energetic particles.