On Vinzent's "if it were so" statements, well those can be called weasel words. But I don't fault him for caution, as the entire genesis of Christianity is speculative. The observations are worth consideration, so I'll go through them and give my take FWIW.
Vinzent's prelude about how the gospels came into being does not account for the ür-Gospel(s). This is already a problem, although he assigns the unpublished prototype Gospel to Marcionites. I am less convinced. The two forms of this prototype, represented by the shared Mark-Matthew, and the shared Mark-Luke, including in Matthew-Mark the feeding of the 4000 set of stories which are duplicates in what appears to be simpler form, indicates these documents had some gestation period prior to the publishing of the Marcionite Gospel. This has serious implications for what follows.
(1) Until the times of Marcion, those who read the Jewish Torah, the Prophets and the Jewish writings ... did so while they regarded themselves as Jews, perhaps as God-fearers, but at least as sympathizers of the Synagogue, whereas other Jews (like initially Paul himself) and gradually the outside world saw in these Jesus-oriented Jews a Jewish branch of its own.
There are several problems with this view. While I think it correct, that these followers of "the way" included, and perhaps were dominated by a nominally Torah and Nevi'im observant factions -referred to as Jews or the Circumcision by Paul- it assumes Jesus was development of a faction within the movement and not common. I think this is incorrect, the split was already made from Judaism before Marcionism erupted. The term Synagogue is actually the Greek for assembly. If by this term Vinzent means that Christians followed the Judae Provencial Assembly and their rulings on legal code (and Torah was like Sharia today, the Law for Jews in the Empire ... not an abstract religious code we think of today) then perhaps I might agree on this point.
(2) Only after the end of the Bar Kokhba war (132-135 A.D.), did the business man and ship-owner Marcion turn (Jesus-)Jews, proselytes, God-fearers, and sympathizers of the Synagoge, as Tertullian claims, into “God-lovers,” re-discovered Paul and collected his letters, probably also collected.. sayings [attributed to] Jesus [or a Christ], and came up with the new literary genre of combining those sayings with narratives to create the Euaggelion, the new angelic edict or law. With the arrival of the new Master and his New Testament, one saw the Old Law of the Creator God replaced, the prophets and their writings no longer valid. For the first time Marcion coined the technical terms “Gospel” for a written document, the “New Testament” (namely 'the Gospel' plus Paul’s letters) as opposed to the Old Testament, termed the movement “Christianity” as opposed to “Judaism” and introduced liturgical rites such as “baptism” to replace Jewish “circumcision” and other liturgical praxis to form a separate religion.
I actually agree with much of this statement. The Bar Kokhba revolt resulted in dissolution of the province Iudaea (Judea) and thus the end of Roman recognition of the Jewish legal code, leaving Jews throughout the empire without a law they could appeal to --- they now fell into the category of provincials and expats in the empire without a legal code, who fell under the law of the peoples (Ethnos). This caused the circumcision problem and led to the Antoninus Pius to amend the Roman legal code to allow Jewish circumcision, although no longer following Torah and applying to all in the household, but restricting it to Jews (their children) only -- similar to the ruling allowing circumcision for Egyptians that was already on the books. Much of the Pauline writing about the Law finds context in the decades immediately after the Bar Kokhba revolt. I am less convinced death and resurrection are from this era, and may have existed prior.
(4) Marcion, however, not only created the basis for the growing new race of the Christians, inspired them with the narratives of Jesus and endowed them with Paul’s letters, he also became the one target with and against whom Christian theology developed, although it took until Irenaeus that the four Gospels were fully credited with Apostolic authority and harmonized with Paul on the basis of Acts.
What I would be willing to say is that Marcion started the evangelism, and turned the Gospel into the tool for that. It is worth noting here that I place Irenaeus in the first half of the 3rd century -- the 2nd century placement is simply too soon for the full acceptance of the four gospel form, ad likely they were not in the form Irenaeus knew until Severus' reign anyway. (There are other problems with 2nd century dating of Irenaeus)
(5) If this were so, Marcion was the initiator and cult-founder of Christianity. He also provided the basis for modern New Testament Studies, while his contribution, on the basis of plagiators and heresiologists, became obscured already during Patristic times.
The diversity in the texts of Marcion as well as the ür-Gospel point to Marcion being one of many, not a a singularity
“’That was excellently observed’, say I, when I read a passage in an author, where his opinion agrees with mine. When we differ, there I pronounce him to be mistaken.” - Jonathan Swift