Are Catholics allowed to entertain that the Old Vulgate might contains errors? In Decrees of the First Vatican Council (1868) in Session 3, Chapter 2 &quot;On Revelation&quot;, items 6-7 state:

The complete books of the old and the new Testament with all their parts, as they are listed in the decree of the said council and as they are found in the old Latin Vulgate edition, are to be received as sacred and canonical.  These books the church holds to be sacred and canonical not because she subsequently approved them by her authority after they had been composed by unaided human skill, nor simply because they contain revelation without error, but because, being written under the inspiration of the holy Spirit, they have God as their author, and were as such committed to the church.

I cannot tell if a Papal Encyclical is the same as infallible statements which are Fides ecclesiastica or ex-cathedra or if they contain such infallible statements.  If not, it appears as though an encyclical statement still represents the end of theological debate, as per Pope Pius XII in Humani generis (which is a Papal Encyclical itself):

Nor must it be thought that what is expounded in Encyclical Letters does not of itself demand consent, since in writing such Letters the Popes do not exercise the supreme power of their Teaching Authority. For these matters are taught with the ordinary teaching authority, of which it is true to say: &quot;He who heareth you, heareth me&quot; (Luke 10:16); and generally what is expounded and inculcated in Encyclical Letters already for other reasons appertains to Catholic doctrine. But if the Supreme Pontiffs in their official documents purposely pass judgment on a matter up to that time under dispute, it is obvious that that matter, according to the mind and will of the Pontiffs, cannot be any longer considered a question open to discussion among theologians.

As I cannot ascertain the difference between an infallible statement and a statement which closes a topic to theological discussion the question is:
Given declarations in the encyclical of Vatican I regarding the Old Latin Vulgate (whether these declarations are dogma or not, I cannot tell), are Catholics allowed to entertain that this translation contains errors, considering that another Encyclical states that encyclical statements close their subject matter to debate?
Are Catholics allowed to entertain that the Old Vulgate might contains errors?
The Old Vulgate Bible translation of St. Jerome is free of errors on faith and moral. Like any translation there may be technical error of nuance of given words or expressions that may have been translated better.
Pope Pius XII declared the Vulgate as &quot;free from error whatsoever in matters of faith and morals&quot; in his encyclical Divino Afflante Spiritu:

Hence this special authority or as they say, authenticity of the Vulgate was not affirmed by the Council particularly for critical reasons, but rather because of its legitimate use in the Churches throughout so many centuries; by which use indeed the same is shown, in the sense in which the Church has understood and understands it, to be free from any error whatsoever in matters of faith and morals; so that, as the Church herself testifies and affirms, it may be quoted safely and without fear of error in disputations, in lectures and in preaching [...]&quot;

The inerrancy is with respect to faith and morals, as it says in the above quote: &quot;free from any error whatsoever in matters of faith and morals&quot; but not in a philological sense:

[...] and so its authenticity is not specified primarily as critical, but rather as juridical.

So yes Catholics may hold that there could be technical biblical inaccuracies in translation within the Old Vulgate, but no outlandish errors.
For many centuries, it effectively was the Bible for countless Christians.
Through long ages in the west, educated people could read Latin but not Greek or Hebrew, and there were few Bible translations in the vernacular available.

The Original Languages
Despite its influential role, the Vulgate is a translation.
It thus does not contain the text of the Bible in the original languages (Hebrew, Aramaic, and Greek).
While it can play a useful role as a translation, it cannot replace the original language texts.
This is an important point, because some Catholics have placed so much stress on the Vulgate that some people have been confused on this point.
Trent’s Statement
To see this, let’s start by looking at what the Council of Trent had to say regarding the matter:
[This] sacred and holy Synod—considering that no small utility may accrue to the Church of God, if it be made known which out of all the Latin editions, now in circulation, of the sacred books, is to be held as authentic—ordains and declares, that the said old and vulgate edition, which, by the lengthened usage of so many years, has been approved of in the Church, be, in public lectures, disputations, sermons and expositions, held as authentic; and that no one is to dare, or presume to reject it under any pretext whatever [Decree Concerning the Edition and Use of the Sacred Books, 1546].
Or, more simply:
[This Synod] ordains and declares, that the said old and vulgate edition . . . be, in public lectures, disputations, sermons and expositions, held as authentic.
“Authentic” in this context means “authoritative.” So Trent is saying that, of the Latin editions available in its day, the old Vulgate was to be considered the authoritative edition for use in lectures, debates, sermons, and expositions.
Note the qualifiers: “out of all the Latin editions, now in circulation.”
Trent isn’t saying anything about original language editions. It’s just talking about Latin ones.
It also isn’t saying that the old Vulgate can’t be superseded later by a newer Latin translation.
Both of these points will be important.
Pius XII’s Statement
In 1943, Pope Pius XII commented on Trent’s statement, writing:
And if the Tridentine Synod wished “that all should use as authentic” the Vulgate Latin version, this, as all know, applies only to the Latin Church and to the public use of the same Scriptures; nor does it, doubtless, in any way diminish the authority and value of the original texts.
For there was no question then of these texts, but of the Latin versions, which were in circulation at that time [Divino Afflante Spiritu 21].
Here Pius XII does two important things.
First, he makes the point we’ve already mentioned—that the Vulgate does not “in any way diminish the authority and value of the original texts” (i.e., the ones in Hebrew, Aramaic, and Greek).
Second, he clarifies that Trent’s declaration “as all know, applies only to the Latin Church.”
This is important because the Latin Church is not the whole of the Catholic Church. - Is the Vulgate the Catholic Church’s Official Bible?

St. Jerome was commissioned by the Church to translate the Bible into Latin. Infallibility of the pope extends to faith and morals, not biblical translations or translators.
Please note I am only dealing with the Old Vulgate in the Latin of St. Jerome. The Duoay-Rheims Bible is an English translation of the the Old Latin Vulgate.