Latin vs. Greek manuscripts
The Douay-Rheims (DRB) is a translation of the Latin Vulgate, whereas the KJV (and most modern translations) follow the available Greek New Testament manuscripts.
The DRB does not contain this word ("dreamers") because it is not present in the underlying Latin Vulgate used for this translation.
The Greek manuscripts all seem to include the Greek participle ἐνυπνιαζόμενοι (enypniazomenoi, "dreaming"), and so translations from the Greek include it.
Translation of the Greek participle
Concerning the translation of this participle in the Greek, the NET translators note that:
The participle ἐνυπνιαζόμενοι (enupniazomenoi, “dreaming”) is adverbial to the pronoun οὗτοι (houtoi, “these”), though the particular relationship is not clear. It could mean, “while dreaming,” “by dreaming,” or “because of dreaming.” This translation has adopted the last option as Jude’s meaning, partially for syntactical reasons (the causal participle usually precedes the main verb) and partially for contextual reasons (these false teachers must derive their authority from some source, and the dreams provide the most obvious base). The participle ἐνυπνιαζόμενοι was sometimes used of apocalyptic visions, both of true and false prophets. This seems to be the meaning here.
The NET translation reads:
Yet these men, as a result of their dreams, defile the flesh, reject authority, and insult the glorious ones (Jude 8, NET).
Concerning modifying "dreamers" with "filthy" as in the KJV (note that it is italicized, indicating that the translators were supplying this word), Bigg notes that:
The meaning involved in the “filthy dreamers” of the A.V. may be confidently rejected, because ... the participle belongs not only to σάρκα μιαίνουσι ["defile the flesh"], but equally to κυριότητα ἀθετοῦοι ["reject authority"] and δόξας βλασφημοῦσι ["blaspheme glorious beings"].
The participle modifies all three following verbs
Schreiner, commenting on versions that translate ἐνυπνιαζόμενοι as "dreamers," agrees that such a translation
is fitting as long as the participle is understood to modify all three verbs [(i.e., defile, reject, and blaspheme)], and the dreams are understood as the basis for the moral baseness of the opponents. They appealed to their dreams as a source of revelation, as a justification for their lifestyle.
Schreiner also notes concerning an alternative perspective:
Others understand Jude as criticizing the interlopers as ignorant, hypnotized, or dreamers, but it is more likely that the opponents justified their moral laxity by appealing to dreams which they believed functioned as divine approval for their behavior.
In a nutshell, "dreamers" is an acceptable way to translate "dreaming ones," but should likely be understood as the basis for the immorality of these individuals, not just a label/name-calling applied to them.
These are likely prophetic dreams/visions
The UBS Handbook further notes the association of "dreams" with prophetic dreams/visions:
These men in their dreamings refers to the godless people mentioned in verse 4. The word for “dream” occurs only one other time in the New Testament, in Acts 2:17, where it is used of prophetic dreams. It is very likely that these people have claimed that through dreams or “visions” (TEV) they receive special revelations from God and thereby gain spiritual insight. Therefore the verse is not suggesting that they perform all these evil acts while experiencing visions, or that they sin in their dreams (which a literal translation may suggest), but that they justify their sinful acts by special revelations they claim to receive from God. If this interpretation is followed, translators may say “These godless people have visions.” In languages that do not distinguish between dreams and visions, the translator may have to use the word for “dream.” However, another possible translation is “special dream.”
A similar example can be seen in Colossians 2:18.
Considering the Greek sentence structure
Following the pattern of Lenski (but modernizing the translation), a wooden translation that over-accentuates the the "μὲν ... δὲ ... δὲ" construction in the Greek, might read as follows:
Yet in the same way these people, too, dreaming, for one thing, defile flesh; for the other, reject authority; for still another, blaspheme glorious ones.
Further explaining this "μὲν ... δὲ ... δὲ" construction, Lenski explains:
Μέν is followed by two δέ, the three particles balance the three criminal acts by holding each item up beside the others. We cannot reproduce such neatness of expression in the English. The participle “dreaming” is predicatively attached to the subject and thereby pertains to all three verbs. In all that they do these libertinistic heretics act like dreamers, unreal images and pictures fill their minds. We speak in the same way when we tell a man who thinks that what is not true is nevertheless true: “You are dreaming!”
The Greek New Testament manuscripts include the participle ἐνυπνιαζόμενοι (enypniazomenoi, "dreaming"), whereas the Vulgate used by the Douay-Rheims (DRB) does not have a corresponding Latin word.
The participle modifies all three verbs (i.e., defile, reject, and blaspheme).
The Greek sentence structure (i.e., the "μὲν ... δὲ ... δὲ" particles) puts each of the three verbs on par as actions these dreaming people (the subject) try to justify.
While a participle in English is traditionally translated with an "–ing" ending (i.e., "dreaming"), this doesn't always adequately convey the versatility and meaning of Greek participles. The English reader might have trouble connecting "dreaming" with each of the three verbs that follows.
The key takeaway is that the individuals doing these things are dreaming (in the sense of prophetic visions), and the author felt mentioning this dreaming was relevant to their behavior. The dreaming is the basis for (or otherwise related to) these people's actions. Any translation that conveys this is satisfactory.
Comparison of various English translations that follow the Greek text
Most of the translations in the question that follow the Greek manuscripts do a satisfactory job of conveying this idea while leaving the precise relationship ambiguous. Examples include (listed from most wooden to least):
- "... these people also, dreaming ..." (NASB)
- "... those dreaming ..." (YLT)
- "... these people in their dreaming[s]..." (RSV, ASV, WEB)
- "... these dreamers ..." (CEB, NKJV, NRSV, HCSB, ISV, NABRE, Tyndale/1526, Coverdale/1535)
Conversely, these translations venture further into the realm of explicitly interpreting the adverbial relationship of the participle for the reader (versus leaving it ambiguous):
- "... these people also, relying on their dreams ..." (ESV, CSB)
- "... these men, [because of || as a result of] their dreams ..." (LEB, NET)
- "... these people—who claim authority from their dreams— ..." (NLT)
Concerning whether the ambiguity is better vs. interpreting the relationship further in the English translation is a matter of preference.
I think these translations that follow the Greek manuscripts all do a decent job of translating the underlying idea.