is it ever proper to put the vocative word first in a sentence?
Yes.
Tityre, tu patulae recubans sub tegmine fagi
siluestrem tenui Musam meditaris auena.
The first sentence of Vergil’s First Eclogue.
It seems to be very unusual in prose, at least if you don’t count exclamations. For instance, the word “iudices” occurs 491 times in Cicero’s works (according to a search of the PHI database), and almost always in the vocative (according to my brief scan), but in the only three sentences where it is the first word it is in the nominative or accusative.
More verses (easier to remember than prose) off the top of my head:
Cui dono lepidum nouum libellum
arido modo pumice expolitum?
Corneli, tibi, namque tu solebas
meas esse aliquid putare nugas.
Catullus 1.
Maecenas, atauis edite regibus, . . .
Horace, Odes 1.1.
Musa, mihi causas memora, quo numine laeso
quidue dolens regina deum tot uoluere casus
insignem pietate uirum, tot adire labores
impulerit.
Vergil Aen. 1.7 ff.
Furi et Aureli, comites Catulli . . .