That's what it's all about in Toobworld. The rest of the TV Universe wasn't put on hold while you watch one particular show. For example, on the same day that Archie Bunker got locked in his basement in Queens, NY, paramedics John Gage and Roy Soto inherited one million dollars on the other side of the country.
Here's another point about Toobworld that should be understood: There should always be a splainin to cover up for any mistakes to be found in TV shows.
With that in mind......
From the TARDIS Data Core Wiki:
Vashta Nerada were swarming, carnivorous beings. The name meant "the shadows that melt the flesh".
The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. Individually, Vashta Nerada were not a threat. They were very small; the Tenth Doctor claimed that some of the dust specks visible in bright sunlight were single Vashta Nerada or small swarms, not large enough to be a threat to most life. In large numbers, they could strip a creature to its bare bones in milliseconds. (TV: Silence in the Library) Vashta Nerada swarms were sentient and two spacesuit-inhabiting swarms developed the ability to communicate with the Doctor through the spacesuits' neural relays. (TV: Forest of the Dead) They also coordinated their efforts and made the spacesuits walk, though with some difficulty; they walked slowly in a "zombie-like" fashion.
The Vashta Nerada lived in the darkness. They cast shadows when they entered the light — even stopping the reach of light from fission lamps. On their own, outside of the darkness, a Vashta Nerada swarm looked like a shadow cast by nothing. They mimicked the shadows of their prey to get close, which meant staying in the light was the only way to escape. Light also slowed them down somewhat, but if the swarms were great enough, the effects of light would be minimised and overcome quickly. If someone had an extra shadow, it was already too late. The Doctor could detect the difference between a Vashta Nerada swarm and an ordinary shadow by using his sonic screwdriver. (TV: Silence in the Library/Forest of the Dead) The Doctor believed they had no weaknesses, and the only thing to do was to run. (TV:Silence in the Library)
That adventure took place in the 51st Century, but the Vashta Nerada were around for millenia before that. Like the Doctor pointed out to Donna, she could see them as the specks in a sunbeam back in her own time.
And I think we could see them in a more concentrated mass back in June of 1966.
"THE GIRL FROM AUNTIE"
When Steed returns home early from a holiday, he finds Mrs. Peel replaced by an actress, Georgie Price-Jones. Subsequently everyone involved in the hiring of Georgie is stabbed to death with double-O knitting needles.
Among those who were involved in grooming Georgie to impersonate Mrs. Peel were the Jacques Brothers - John, Paul, George, and Fred, theatrical costumers. After Steed and Georgie discovered their bodies, their actual auntie, Aunt Hetty, arrived to pick up her book of knitting designs.
She was oblivious to the fact that her four nephews were lying there dead and I think it was somehow covered up so that she was never informed of the truth. And how did that happen?
Their bodies were gobbled up by the Vashta Verada, leaving no proof of their deaths.
Take a look at these four screencaps from that episode of 'The Avengers':
From the Movie Mistakes site:
When the old lady enters the costume shop to retrieve her knitting design book, an equipment shadow sweeps across the lower right of the screen, then moves back to the left.
Okay, that's fine for the real world, but why were those shadows moving within the reality of Toobworld? They had to be Vashta Nerada. Luckily Aunt Hetty, along with Steed and Georgie, were able to get out of that room before the Vashta Nerada decided to feast on them.
So there's the "equel", the story that was taking place from a different perspective. Once Steed and Georgie moved on to the next stage of their investigation, the Doctor - who knows which incarnation? - arrived with his Companion(s) and convinced the carnivorous collective to get into the TARDIS and be relocated back to their homeworld. Perhaps they were the first colonists to that world of the Forest.