Thanks for sharing your doubts with the Lingvist community!
Vosotros/as and ustedes have roughly the same meaning but take different grammar persons:
vosotros/vosotras --> 2nd plural (estáis, coméis, decís, etc.)
ustedes --> 3rd plural (están, comen, dicen, etc)
Ustedes decís – Ustedes dicen
Vosotros dicen – Vosotros decís
Please, remember to check the grammar person of the main verb before choosing the corresponding personal pronoun. Do not worry if you can't identify it at first: just introduce the form you think correct or click on "Reveal" so that the correct form will show up, as well as the full conjugation table for that specific verb so you will be able to check the right forms.