What separates a v45 from a v51?
Almost nothing. Literally. V45 is 99th percentile and V51 is 99th percentile. Unless you're hoping to work for a test prep company - or a company that makes the real standardized test - there's no concrete distinction. :)
The difference is a few questions here and there. V51 is perfect or almost perfect - zero or maybe only 1 question wrong. V46 is a few questions wrong. Toss in a vocab word you happen not to know, an obscure grammar rule, a really tricky trap answer or a careless mistake, a little fatigue getting to you... or get rid of all of those things and be In The Zone and maybe you'll get the 51 instead.
The reason you don't see articles about going from 45 to 51 is that this is rarefied air. People who work for test prep companies and literally study Qs for hours a day for a living - well, that's how they do that. Want to come sit by me 8 hours a day for the next 3 months and learn? Watch me pick apart one question for 15 minutes and then write another new one myself - one that looks completely different yet shares some fundamental reasoning or something like that? Come on over. (And if you really do want to, then hey, maybe you do have a standardized testing career in front of you. :)
Are you retaking the test just for this? Or are you retaking it for quant and just also want to set yourself a verbal challenge since you're retaking anyway?