The connotation of pretty much any adverb applied to "impossible" is that there is a known circumstance or set of events that would produce the "impossibility", but that those circumstances are so improbable as to not be worth consideration. "Virtual" itself is defined as "very close to being something without actually being it", and so any term you can think of with a similar definition would be synonymous.
In addition, there are many words or phrases with a similar meaning, "incapable of being considered":
inconceivable, unthinkable, vain, hopeless, infeasible, unachievable, unattainable, out of the question, insurmountable, useless, impassable, inaccessible, unworkable, preposterous, absurd, illogical, incredible, untenable, implausible, unimaginable, unobtainable, not to be thought of, hardly possible, beyond the bounds of possibility, impracticable, improbable, extremely unlikely
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Some other ways: