This is not a duplicate question. The question isn't how much of it has been lost, but how it was lost.
Vedas had oral tradition and has been preserved by pure memory for thousands of years. How can this be lost? There were always brahmanas, and there are still brahmanas, maintaining guru shishya parampara. There has been brahmanas memorizing skillfully everything for millennia. After all that, how did Vedas ever get 'lost'? From Puranas it seems that even by the time Puranas were written some of it were lost.
Can it be precisely known exactly how much amount of Vedas has been lost?
Did it get gradually lost? Did the mechanism of memorizing every verse and transmitting it orally fail? I have heard some people say that many Islamic invaders came and killed brahmanas and burned down libraries and this is where it got lost, but all brahmanas weren't killed, most brahmanas were alive and also the books burned were mostly Buddhist in Nalanda. Also, as I said it seems Vedas were being lost even before Islamic invaders. So how exactly did such huge volumes get lost? Also in which period did they get lost? Did they mostly get lost early during 1000BC, or did they get lost during medieval era? Did Buddhism conversions contribute to Vedas getting lost?