Vaticanus is thought to be earlier because it does not have the
Eusebian Canons, unlike Sinaiticus, which has them from the first hand.
The Eusebian canons were added to Sinaiticus by the first hand, but not added to the whole document, as they aren't in Luke.
It is posited by Skeat that both Sinaticus and Vaticanus were both the immediate consequence of Constantine's order for 50 bibles for Constantinople, and originated in the same scriptorium, although it is conceded that Sinaticus never reached Constantinople, after it was abandoned, most probably in Caesarea.
However much of the history of Vaticanus is speculation. Vaticanus was likely soon viewed as defective (as was Sinaiticus) and so was left unused for a long period, whereever it ended up, prior to its defects being remedied at a much later date (possibly in Constantinople).
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Skeat Collected Writings by Elliot JK (Dating and origin of codex vaticanus)
p.286
Among Skeat’s persuasive arguments is the constant message that
no-one working in this area should forget that Codex Sinaiticus and
Codex Vaticanus are from the same scriptorium. The common ori-
gins of Codex Sinaiticus and Codex Vaticanus have been regarded
as axiomatic from the days of Tischendorf through Lake to the present
and no responsible New Testament scholar should ignore this
fact. Among his proofs are:
i) The very close resemblance of the colophon design at the end
of Deuteronomy (in Codex Vaticanus) with that at the end of
Mark in Codex Sinaiticus.15 [This Skeat identifies as his strongest
argument and one which must be understood and recognised.]
ii) Possibly Codex Sinaiticus shares a scribe with Codex Vaticanus.
Two of their hands may be identical. This is a disputed point
because the re-inking of Codex Vaticanus at a later date (probably
ninth-tenth centuries) makes it difficult to examine carefully
the hand of the original scribes. Tischendorf thought hand D of
Codex Sinaiticus was the same as hand B of Codex Vaticanus
but Milne and Skeat argued16 that the closest resemblance was
between scribe D of Codex Sinaiticus and scribe A of Codex
Vaticanus and that, even if they are not the same, “the identity
of the scribal tradition stands beyond dispute”. Cavallo agreed
with Milne and Skeat. However, this is not a point Skeat himself
would now wish to dwell upon.
[We must remember that the colophon designs were not reinked,
although the lettering was.]
iii) Another relevant consideration is the fact that Vaticanus and
Sinaiticus both end their text of Mark with the same verse. One
of the features of Codex Sinaiticus and Codex Vaticanus is that
they, virtually alone among New Testament manuscripts, end
Mark at 16:8 (even though it is plausible that the scribe of Codex
Vaticanus was hesitant to do so.)17 Sinaiticus does not provide
any evidence for the continuing of the text after verse 8, and
did not do so even before the re-writing of the bifolium, the
error which provoked the re-writing being in the text of Luke 1.
[Skeat posits that Vaticanus was sent to Constantinople from Caesarea.]
p.291
As far as competing places of origin for the composition of the
two manuscripts are concerned (i.e. other than Caesarea), the strongest alternative
(and the one favoured in the introduction to the new facsimile) is Alexandria.
That is often based on the several grounds. These are noted below with counter-arguments attached:
1) The suggestion has been made that Codex Vaticanus was one of
the Bibles sent from Alexandria by Athanasius to Constans has
already been referred to. But if Vaticanus had been sent from
Alexandria to Niš (Serbia) we need to ask how, when and why it got to
Rome in the fifteenth century.
2) The text of Vaticanus resembles the text-type of certain third century
Egyptian manuscripts, notably P75. But this need not be a
decisive argument in favour of Alexandria and against Caesarea.
As Zuntz reminds us, Caesarea was a centre of Alexandrian
scholarship—the two cities were not so far from each other: we
need think only of the link from Origen through Pamphilus to
Eusebius himself. Also to be remembered is the fact that manuscripts
older than Vaticanus and Sinaiticus are papyri, which virtually
all come from Egypt. We do not have comparable third century
witnesses from other places, such as, for example, Caesarea.
3) Hexaplaric influences in Vaticanus such as the addition of obeli
and asterisks in Isaiah, Zechariah, Malachi and Jeremiah are sometimes
given as evidence of an Egyptian provenance. But they
reflect only Egyptian influence that could plausibly have reached
Caesarea through the person of even Origen himself.
4) Earlier arguments, by Lake and others, emphasise that certain
features of the script of Codex Sinaiticus are Egyptian (the alleged
Coptic mu, a cursive xi and a strangely formed omega) but these
have been dismissed by no less an authority than Cavallo and
by Milne and Skeat as not decisive.