Veddas were mixed with Singalas and also Tamils in East and North.
So it goes both ways. It is hard to prove waht exactly what happend.
There are no documents to prove this, It is like Rawana, Seetha story, It could be a fict
In this modern world Some people still believe that No one landed on earth, although it is not true. There for there are some odd ones like you who don't believe in anything.
There are reported veddha sinhala marriages.
But up to my knowledge there are no records to show that veddahs are mixed with any other race.
You can notice that there are no Hindu or Muslim weddas But they all are Buddhists
With related to your Ravana story what I can say is
yes there are no written proofs to prove that
But thre are places of incidents those are exciting giving proper evidence
People in Srilanka believe that king Ravana is a Hela king
Who invaded India
And people who do not have a proper connection with srilanka believe the same story.
If you want I can describe them in detail and Show you that there is a valid point behind the reality of a Ramayana and existence of Ramayana
Best examples are
A place near Galle With completely different natural setup from the rest of the area and having very high gravity
IF it is a fiction how people who live 2000 miles away believe the same thing in the same manner