Believed to have been written in the 4th century, it appeared in the Vatican Library in its earliest catalog of 1475.
Believed to have been written in the 4th century, it was found (shown to) by Constantin von Tischendorf the Monastery of Saint Catherine, at the foot of Mount Sinai in Egypt, in 1859.
Both of these Codexes are part of the Alexandrian text-type family, also called Neutral or Egyptian. Westcott and Hort used this family and relied heavily on Vaticanus and Sinaiticus for their 1881 The New Testament in the Original Greek.
Before Karl Lachmann first in 1850, and Westcott and Hort’s Greek, the Byzantine text-type was used, as through the Reformation with Tyndale and the Bibles which followed.
Interestingly virtually all modern translations, since the Kings James Bible, have come from the Alexandrian text-type family. Or I don’t know of any from the Byzantine text-type.
I have heard countless times in reading the phrase “oldest and best manuscripts” by the lovers of the Vaticanus and Sinaiticus. What exactly does that mean anyway?
Things that I think need to be taken into consideration:
 Vaticanus and Sinaiticus were written on vellum, whereas the Byzantine text-type was papyrus, which could explain why they have found earlier “dated” copies.
 Vaticanus and Sinaiticus were not used for over a thousand years, whereas the Byzantine text-type has been used through clearly visible times of God moving.
 Vaticanus and Sinaiticus are believed to be from in Egypt (where scripture claims for God’s people not to go), whereas the Byzantine text-type is believed to be from Antioch (where we see the first use of Christians).
If virtually all modern translations use the Alexandrian text-type, which are supposedly the “oldest and best manuscripts”, why is it that they vary (some more than others) more often than the Byzantine text-type translations (within themselves), they have not caused such great awakenings such as found with the Byzantine text-type, they claim to have made it easier to read and understand but the churches nor its people (as a whole) have come closer to God with a richer understanding and love for the things of God?
Why is it that no new translation wants to touch the Byzantine text-type when history has proven it has done wonders for the cause of Christ?