Learn Italian: Vendiamo leggii e vecchi planetari. - We sell reading stands and old orreries.

This perfectly demonstrates the rule for masculine nouns ending in -io. In the singular leggio, the stress falls on the 'i' (leg-gí-o), so the plural keeps both 'i's: leggii. In planetario, the 'i' is unstressed (pla-ne-tà-rio), so the plural drops the 'o' and ends in a single 'i': planetari.

Does the adjective vecchio follow the same unstressed -io rule?

Yes! The singular is vecchio (VEC-chio). Because the stress is on the first syllable and not the 'i', it follows the exact same rule as planetario and forms its plural with a single 'i': vecchi.

Why is the adjective vecchi placed before the noun planetari here?

Vecchio is one of a handful of common adjectives (along with words like nuovo, bello, and bravo) that usually precede the noun. You could say planetari vecchi, but placing it after the noun would put a heavier, more literal emphasis on the fact that they are old and perhaps worn out.

Could planetari also mean 'planetariums' here?

Absolutely. Planetario translates to both 'planetarium' (the building or room with a dome) and 'orrery' (a mechanical model of the solar system). Given the context of selling them alongside reading stands, tabletop models (orreries) are much more likely!