I have a copy of Quomodo Invidiosulus... and I can tell you that it is a very difficult text, primarily because of its vocabularly (as has been mentioned). I don't think anyone who isn't an avid cultivator of Living Latin like Tunberg could read it without looking up a lot of words. To expand his lexicon to fit modern topics, and to approximate Seuss's rhyme scheme, he drew upon Latin words from ancient times through today. Learning vocabulary is difficult enough when you're limited to one author or time-period; taking on all of Latinity at once will overwhelm you and isn't worth it at an early stage. Read Tunberg's translation as best you can and enjoy it, but I wouldn't bother studying the vocabulary at this stage.
Librum "Quomodo Invidiosulus..." etiam habeo atque tibi dico hoc difficillimum esse textum, praesertim numeri vocabularii causâ (ut suprâ est dictum). Nego aliquem non Latinae Vivae cultorem, sicut Tunberg ipsum, hoc librum sine auxilio vocabularii crebro legere posse. Ut res modernas narrare imitarique Seussi homeoteleuta possit, Tunberg ubique verba Latina carpsit non solùm priscis è temporibus sed etiam hodierno è die. Si vel unum auctorem vel aetatem legis, verba multa discere difficile est; si nimis subitò in omnia ruis, rutus eris. Magis latinè secuturum est...
I was unaware of those vocbulary lists for Lingua Latina--thanks a lot! Obiter, I wonder if it's possible to estimate one's knowledge of Latin vocabulary. You often hear estimates (frequently apocryphal) about the vocabulary of an English speaker these days. How many Latin words is one expected to know at a year of study? After four years? How many words do you need for 80% comprehension of a written classical text? 99%? This is all regarding sight recognition; being able to actively use Latin vocabulary is a whole other beast.
Horae quidem cedunt et dies et menses et anni, nec praeteritum tempus umquam revertitur nec quid sequatur sciri potest. Quod cuique temporis ad vivendum datur, eo debet esse contentus. --Cicero, De Senectute