quaedam and nōn quaedam
Most fundamentally, quīdam/quaedam (plural) means "certain ones", i.e. some instances. But I gather (I'm not an expert) that unless some adjective of quantity is supplied, it usually suggests a small, manageable number—"a few".
Googling "quaedam dicenda" turned up enough examples that that itself appears to be a common phrase: "some things that need to be said", "a few things that need saying". Celsus, De Medicina, I.3.17, has a typical example:
Cum vero inter extenuantia posuerim vomitum et deiectionem, de his quoque proprie quaedam dicenda sunt.
But since among methods of weight loss I have placed vomiting and purging, about these specifically there are a few things that must be said.
Seneca, Epistle XCV.36, has a negation of quīdam:
"Quid ergo? Non quidam sine institutione subtili evaserunt probi?"
"What? There aren't any [people] who have turned out well without refined instruction?"
If we think of quīdam as "a few", that sentence might be better understood as "Aren't there plenty of people who …?" I believe this is the litotes you were looking for.
Varro, De Agri Cultura II.11.4, negates a negation of quaedam:
Neque non quaedam nationes harum pellibus sunt vestitae, ut in Gaetulia et in Sardinia.
There are even some nations [suggesting in this context that they are especially crude nations] that are clothed with the skins of these [i.e. goats], such as in Gaetulia and Sardinia.
This example seems to work oppositely from the others. Nōn quaedam here would seem to mean "not any", which gets negated into "some".
vix and vix non
I understand vix to mean almost but not quite negation: just barely surpassing some threshold. Here's an example from Tacitus, Historia Augusta 14.5, which also illustrates quīdam indicating a small number:
Duo igitur principes una exstiterunt domo, quorum alter sex mensibus, alter vix duobus imperaverunt, quasi quidam interreges inter Aurelianum et Probum …
And so two princes arose from one house, of which one ruled for six months, the other for barely two [months], like a couple of regents between Aurelian and Probus.
Googling "vix nōn omnis" and similar phrases turned up plenty of confirmation that vix nōn X works logically: "almost but not quite X", i.e. falling just below the threshold for X. Joannis Petri Frank, De Curandis Hominum Morbis, Book VI (1821), p. 440:
… quia pulmo vix non omnis consumitur, dolore, tussi, febre, quae somnium impediunt, corpus alterant …
… because the lung is nearly all consumed, pain, coughs, and fever, which impede sleep, alter the body …
An entry in A Grammatical Dictionary of Botanical Latin has this example and translation:
nobis (si perianthii indolem rite intelligimus) in omnibus nisi staminum structura cum Bixineis convenire videntur, et vix non carum subordinem constituent (B&H)
by us (if we correctly understand the nature of the perianth) in all except the structure of the stamens, seem to correspond with the Bixineae, and hardly constitutes a worthwhile suborder.
In this passage, vix non seems to operate as a synonym for parum, which I understand to mean the opposite of vix: "so meager, it's not even close". Perhaps in New Latin, people started using vix non this way. This might explain's Fux's usage. But see below; I don't think we need to reach for this. (Perhaps the author was straining to avoid the rhyme of parum carum.)
If we take vix nōn as a sort of modifier for quaedam, we get "slightly less than a few". That doesn't seem to make much sense in context.
If we take vix as a modifier for nōn quaedam in the sense of "not a few", i.e. "plenty", then we get "slightly more than plenty". In context, this makes sense—as understatement. It's like saying in English "not just a few". In Latin, it's an understatement on top of a litotes (itself a kind of understatement).