Vuestro/a...

[Sancho Panther]

is the adjective indicating plural ownership; is it employed in both formal and informal usage?

[AngelicaDeAlquezar]

:thinking: As far as I know, "vosotros" and its conjugation and pronouns are nowadays for informal use only.
The formal use of "vos"/"vosotros" with the conjugation and pronouns for "vosotros" is archaic, and it could be used for one person or many:

- Señora, vos sabéis que vuestra presencia me honra.
- Os doy la bienvenida, Vuestra Excelencia.
- Caballeros, permitidme mostraros vuestra nueva morada.

[Sancho Panther]

So it's singular as well as plural then?

[AngelicaDeAlquezar]

In the archaic use only.
In current use it's only informal plural. :)