I have seen two different constructions to express "far too early" : "allzu früh" and "viel zu früh"? Are they interchangeable or is the meaning/usage different?
There's a difference in meaning. "viel zu früh" is just a stronger version of "zu früh" (too early). There is a clear threshold defining what level of "early" is still acceptable. "viel zu früh" is far beyond that.
Im 7 ist zu früh.Um 5 ist viel zu früh.
"Allzu früh" usually doesn't imply such a threshold, and it is not necessarily expressing that something is too early. Or at least that's not the focus. It just expresses that it's very, very early (which might also mean that it's too early). I think "overly early" is a good fit, though the common translation is "too early" (according to Linguee.com) Here some example I found on the web:
Bumerang Babys. Neugeborene nicht allzu früh entlassen.
In Barcelona hingegen wollte ich nur nicht allzu früh ausscheiden. (Kontext: some fencing tournament)
Man will ja den Abend nicht allzu früh beenden müssen.
Hoffe muss morgen nicht allzu früh aufstehen ;) (from Facebook)
You cannot use "viel zu" in any of those examples without changing the tone because you'd be shifting focus to the distinction between early/too early.
The standard expression is viel zu früh. Allzu is not technically wrong, but definitely outdated / poetic. Also, it only really works in general expressions:
Allzu früh verschied der Dichter ... the poet died far too early
In a normal sentence it would sound incredibly stilted:
Wir kamen viel zu früh (not: allzu früh) zur Party.