Taken from the link below.
http://www.chuckhawks.com/wby_vanguard_rifles.htm
The two position Vanguard safety locks the bolt closed in the "safe" position, just like a Mark V safety, but it is not as quiet in operation and is shaped differently.
So either Chuckhawks is wrong or the three position safety is a very recent change.
I have fired several vanguards myself with two position safeties,butnot in the last couple of years.