A poem with a 10-word vocabulary:
That jumble of words, in a sense, for my minding.
Of minding my words, in a jumble, that for sense.
For sense my jumble, that. In words: a minding of.
Sense of my words? In that, for a minding jumble.
For jumble my words of a minding, in that sense.
….
….
A wordy mind. In the jumble. For senses of myself.

A poem with a 7 word vocabulary:
People used small terms, with a cost.
Terms: used. With a small cost. A people.
Small people used terms with a cost.
Used with a people, small terms cost.
Used people. A cost with small terms. 
…..
…..
People use costly terms with a smallness.

A poem with a 5- word vocabulary

Long phrases are too much,
Phrases are much too long.
Are too! Much long phrases.
Much long? Are phrases too.
Too much are long! Phrases.
….
….
Many are longing. To phrases!