Q:

Adjectives' morphology

Why is "extravagant" a word and "vagant" isn't?  
Here is what OET has to say on "extravagant":

late 14c.,  in constituciouns extravagaunt, a term in Canon Law for papal decrees not originally included or codified in the Decretals, from Medieval Latin extravagantem (nominative extravagans), present participle of extravagari "wander outside or beyond," from Latin extra "outside of" (see extra-) + vagari "wander, roam" (see vague).  

"Vagant" does not display any results as it is not a word.  
What gave this question away is Jorge Luis Borges in "The Library of Babel".  He mentions a discovery that brought "extravagant happiness".  Being that his craft rarely includes prefixes, I had this corollary (as his translator has put it) question knock on my head, which I thought would be clear by itself without additional explanations for it applies to numerous contexts.  

A:

Extravagant came into Middle English from Latin extrāvagantem (from a verb, extravagari) and French extravagant and survived to the present day (OED, "Extravagant, adj.  and n."). 
Vagant independently came into English from the Latin vagans (from a verb, vagari) and Old French vagant, meaning wandering or roaming, but it did not survive into modern English (OED, "vagant, adj."). 
Sometimes when two related words come into English, one survives and the other doesn't.  For instance, the adjective "melliflous," meaning eloquent or (more literally) flowing like honey, has survived into modern English, but the noun "mel" (honey) is hardly used.  It's difficult to say why that happens; historical language development is arbitrary. 
In this case, English listeners tend to understand extravagant not as extra- (or outside) vagant (as one might more recent formations like extramarital, outside marriage), but as its own concept.  If you remove extra-, if people understand it at all, they may construe it as "just vagant," treating the extra- and vagant only in reference to the meanings of extravagant they already know:

My parties could never be described as extravagant — so does that mean that they’re just vagant? ( Saturday Evening Post)
Would you be saving for an extravagant vacation?  I use the word “extravagant” carefully.  If you’re in debt, you’re saving up for that true expense, a vacation, could you whittle that down to not be extravagant but just “vagant”?  Did you see what I did there?  Extra-vagant.  I’m not sure if that works, but the question remains: do you pay off your debt or save money first? ( You Need a Budget)

