Correct. The amount of Wikipedia pages at any given moment with active vandalism is vanishingly small. The only time I have ever stumbled upon vandalism is as part of my work as a volunteer there actively looking for such cases. Looking at my feed of possibly problematic changes at the moment, about 3 entries are appearing per minute with the most recent revert being just 2 entries ago. It is significantly worse while school is in session in my experience, but vandalism very rarely lasts long. Talking to people, people frequently confess to vandalising wikipedia at one point or another. When I ask them "how long did it survive" they tell me answers ranging between "a few moments" to "5 minutes." So to answer your question, I believe it is unlikely the average person has seen vandalism on the site barring those looking at their own shit.