In The Return of the Shadow and The Peoples of Middle Earth. I'm just researching it on the wiki following the sources they cite.
Thanks for the pointer; that's enough for me to find it.
In
The Return of the Shadow, chapter 12 ("At Rivendell", covering what became "Many Meetings" in the published LotR), pages 214 - 215 have a paragraph on the subject:
... long after the publication of The Lord of the Rings, my father gave a great deal of thought to the matter of Glorfindel ... He came to the conclusion that Glorfindel of Gondolin ... and Glorfindel of Rivendell were one and the same: he was released from Mandos and returned to Middle-earth in the Second Age.
The Peoples of Middle-earth returns to the subject: chapter 13 ("Last Writings") starts with six pages on Glorfindel, including two essays, which are evidently Tolkien's "great deal of thought" on the subject. These seem to have been written in the last year of Tolkien's life.
It also says that Tolkien decided Elf names were unique
The second essay in
The Peoples of Middle-earth includes this:
This repetition of so striking a name, though possible, would not be credible. No other major character in the Elvish legends as reported in The Silmarillion and The Lord of the Rings has a name borne by another Elvish person of importance.
As I read this, Elf names are not necessarily unique, but Tolkien decided (very late in his life) all the important ones are. I think Glorfindel is the only case affected by this.
Morgoth was more powerful than Sauron and Fingolfin managed to slice that guy pretty bad. Even if he did kind of get stomped as well.
Morgoth was the single most powerful Vala, and hence the most powerful being in Middle-earth. So I think Fingolfin was doing pretty well to wound him even once, let alone eight times.
When Sauron disguised himself as a vampire, what exactly is a vampire in Tolkienology and what does it look like?
I think you're thinking of Thuringwethil (Sauron's messenger) rather than Sauron himself. She only appears in "Of Beren and Luthien" (when Luthien takes her shape to enter Angband) and is described as "a bat-like creature ... with creased wings" and as having "great fingered wings ... barbed at each joint's end with an iron claw". Basically a big, scary bat.