Flashsides occur when a vampire is under great stress and so will not remember events but see events happening elsewhere or in different realities. These can also bleed out and affect the people around the vampire.
On Roundworld, a person subjected to an extraordinary amount of stress or mental/physical trauma over a short period of time might, later in life, manifest the psychological disturbance known as PTSS - Post-Traumatic Stress Syndrome. The theory of PTSS dictates that the individual copes with the overload of stress by burying it deep down in the psyche, whence it will periodically arise in the form of brief, intense periods of reliving the original incident, or incidents, from which the psyche has tried to retreat. These intense moments of whole-body memory, in which the sufferer will relive the incident as if it is actually happening, are often referred to as flashbacks. These are common in members of armed forces, or police/rescue workers, or survivors of catastrophes, who have witnessed what might be described as the extremes of human experience in the course of their daily work, or simply by getting on the wrong plane/tube train on the wrong day. On a mundane level, if this could be called "mundane", this might be one of many reasons why Samuel Vimes took to drinking after many years in Watch service - he'd just seen too much, including, latterly, a recurring flashback of the little pilot light at the back of a Noble Dragon's throat, which would in other circumstances have signalled The End and a brief sympathetic interview with Death. (Substance addiction is also an indicator of undeclared or denied PTSS in service and police personnel around the world).
As things are slightly different on the Disc, vampires manifest a different set of symptoms. Vampires are creatures of heightened sensitivities at the best of times. If not actually psychic, they have a well-developed intuitive instinct for knowing if the immediate future might hold, for e.g., a sharpened wooden stake or other inimical object. This, in a world where many people quite unreasonably object to having their lifeblood involuntarily consumed, and where humans have always outnumbered vampires in much the same way and for much the same reasons why large vegetarian animals have always outnumbered lions, could be viewed as a well-honed survival mechanism.
It has been noted that the personal space of a vampire radiates out quite a lot further than that of humans. This is handy, as any creature stepping into that personal space might feel the vampire's charisma and natural persuasive charm speaking to them, in terms of just undo that shirt at the collar for me... oh, and do please feel free to take off that holy symbol you're wearing, thank you so much.... But if for any reason, a vampire is under stress or a new set of learnt reflexes are breaking down, or if the obsessive need, however that vampire might define it, is frustrated, then the intimate personal space of a vampire can radiate out for up to ten miles around his or her body. And that's only in our physical dimensions.
As is evidenced with Maladict in Monstrous Regiment, her personal space in a time of crisis not only forces a different reality on Polly Perks and other members of the Last Detail, it has also reached out into a different reality and brought back souvenirs... or just conceivably, in a manner not unlike Rincewind and Twoflower ending up on board a Roundworld airliner in a time of great stress, temporarily broken down the walls of reality and taken them somewhere else. This, then, is the Flashside, in which a little bit of Borogravia very briefly becomes Vietnam-like, right down to the helicopters. You can almost hear Jim Morrison intoning the soundtrack...
However, this being the Discworld, the half-glimpsed helicopters take on the form of those marginal sketches and models made by Leonard of Quirm, suggesting that the afflicted vampire is fleshing out something she saw or read about on her own planet, rather than ours. But then, that doesn't explain why the atmospheric effects are consistent with a helicopter's downdraught, why the sound of the airscrew is exactly what you'd expect to hear, and why the hallucinations float and drift in a recognisably helicopter-like way. None of this can be explained by having read a book by Leonard or by having once seen one his models... and somewhere on the ground, there is a hint that Angua von Überwald, in her werewolf form, is also watching and confused by all this. Not least that an Überwaldean forest and a Vietnamese jungle are winking in and out of each others' planes of existence...
In Piers Anthony's excellent Tarot trilogy, Planet Tarot is unique in that all attempts to establish human colonies there have failed. It appears that collective delusions have rampaged through the humans. Worst, the nightmare visions reported by survivors have actually been filmed... indicating they have some sort of objective reality. Warrior-priest Father Paul is sent to find the Answer. Going into the unreality fields with members of the human colony, he discovers that the illusion is made real, and directed, by the strongest mind in the group, and generally comes out of that person's negative memories, past neuroses, fears, and bad experiences. Which makes a vision dictated by the group's resident Satanist not only scary, but painful and dangerous... Brother Paul is subjected to Tarot-enhanced, near-lethal PTSS-flashbacks to the very worst times in his life as gambling shark and drug addict, and is changed by the experience.