What are other ways to say "virtually impossible"?
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.
Almost surely impossible - This is from math parlance, in which the phrase "almost surely" has a specific meaning; that the event described has a theoretical probability of one (here, referring to the probability of the "impossible" event not occurring). That doesn't mean the opposite (the "impossible" event) can't happen, just that the chances of it not happening are (N-1)/N for a "large" N (approaching infinity).
Practically impossible - The connotation that it is "impractical" (not worth the cost) to plan for it to happen. Doesn't mean it can't, but the odds are so slim the costs and benefits of expecting it aren't worth it.
Nearly impossible - "Nearly" is a synonym and possible definition of "virtually".
All but impossible, more or less impossible, nigh impossible, near impossible - ditto.
It has a snowball's chance in hell - Slightly colorful and well-known, this one's pretty self-explanatory.
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
KeithS mentioned some possibilities but seems to have blithely skipped over one of the best modifiers, vanishingly, as found in phrases like vanishingly small, vanishingly rare, and vanishingly probable. For examples like the following, see Edwin Ashworth's answer to What word is used to describe an event that will never occur?:
The probability that they coincide by chance is vanishingly small," said coauthor Alan Linde, also of Carnegie.
'The number of complaints which are substantiated is vanishingly small....' (Huddersfield Daily Examiner)
(Ashworth calls the latter an example of misuse, as no fixed finite number can be vanishingly small.)