This answer is at the level of first year college physics.
Two concepts have to be cleared up here: "virtual particle" and "particle antiparticle virtual loops" .
Here is a first order Feynman diagram of electron positron annihilation.
We read it as follows: an electron emits a real photon at the round vertex and becomes a virtual electron, and meets the incoming positron, the quantum numbers,lepton, are equal and opposite and they annihilate into the second real photon . The schematic has a very strict mathematical interpretation, one to one with constants and integrals over the variables, and is used in calculating the probability of an electron annihilating on a positron and vice versa.
The "virtual" electron/particle carries all the quantum numbers of an electron but not the mass, it is off mass shell, because it represents an integral over a propagator function with the energies entering variable within the limits of integration.
Here is how the muon decay can be calculated using the prescription of the first order Feynman diagram
Note the virtual W which when real has a mass 80.4 GeV, the muon has a mass of 105.6MeV. The W is very off mass shell in this decay for energy conservation to work on vertices.
The particle antiparticle virtual loops are a mathematical concept coming from the calculations and representing the ground state of the vacuum. It is also connected with the Heisenberg uncertainty principle,
that states that even if we say there is zero energy, there is an uncertainty related to the time in which we make the statement, and this can be drawn as virtual loops of particle antiparticles , i.e. a mathematical formula exists to represent this, virtual loops of particles antiparticles. They are unmeasurable as they have no external lines.
This is what is used in the Hawking radiation arguments, where the virtual loop at the event horizon has one particle interacting with the gravitational field and energy is supplied by it so that the other becomes real and exits as real.
But can the virtual particles be 'boosted' into real particles by other fields, such as an intensely strong electric field? And if so, how strong would the electric field have to be?
With the above background it is evident that virtual particles turn into real when they acquire mass and keep their quantum numbers:
The photon hitting an electron and exchanging a virtual electron with the field of some atom (through a virtual photon or Z) creates a pair by the electron becoming real.
In some cases the existence of the vacuum and the virtual pairs is important in the calculations their existence can affect the result of the calculation adding a fine structure.
A real photon cannot hit a vacuum loop and generate particles: because of momentum conservation two particles have to become real and they will have a rest mass system, whereas the gamma does not have one, in all systems it travels with c velocity.
It is not the intensity of the electric field that will be the problem, but the fact that virtual loops are higher order corrections, which means that the electromagnetic coupling constant enters with high exponents in these loops, and the probability will be very small. In a sense the last diagram is a previous virtual loop that became real .