Could you please explain what the difference in usage is between through and via, which sounds like a Latinism?
Are they completely interchangeable?
Using via as a preposition in English is of comparatively recent provenance. It has substantially fewer primary senses, and therefore available uses, than does through. The OED gives only two main senses for via as a preposition, which I include here with a few of each one’s later citations:
Here are some examples of through (taken from the OED’s citation list for that preposition) where you could not substitute in via in its stead:
In contrast, in these examples from the same source, one perhaps might be able to make that swap:
So even though though there are a few places where you can use via or through — or else via and by — interchangeably as prepositions, there are many others where you cannot.
Finally, it should be noted that there are substantive, adjectival, and adverbial uses of both words, and that these non-prepositional uses are never interchangeable.
tchrist's answer says it all about the definitions of these words.
On the matter of usage, I'm sorry to see via being used in place of through more and more. I put it down to a desire to appear educated and sophisticated, but it backfires, and comes across as pretentious or betrays a lack of care in using the language.
Via has a well established usage meaning to go from one geographical location to another by way of a third. That's how the Romans used it. More recently it was reserved for travel contexts and remains useful in this sense, for example a Northern line train (on the London Underground) going to Morden via Bank.
But for all other uses, through will do perfectly well. It sounds more fitting in our Anglo Saxon tongue and it's shorter (in terms of syllable count) so easier to say, hear and process in the brain. Look at tchrist's last set of quotes that might allow via to substitute for through: in none of them would the substitution improve the flow or clarity or aesthetics of the sentence.